Historic and Modern Air Pollution Studies
Conducted in Utah
Judy Ou 1, *, Cheryl S. Pirozzi 2, Benjamin D. Horne 3,4 , Heidi A. Hanson 1,5,
Anne C. Kirchhoﬀ1,6, Logan E. Mitchell 7, Nathan C. Coleman 8and C. Arden Pope III 8
1Huntsman Cancer Institute, Cancer Control and Population Sciences, University of Utah,
Salt Lake City, UT 84112, USA;
firstname.lastname@example.org (H.A.H.); Anne.Kirchhoﬀ@hci.utah.edu (A.C.K.)
2Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Utah,
Salt Lake City, UT 84132, USA; Cheryl.Pirozzi@hsc.utah.edu
3Intermountain Medical Center Heart Institute, Salt Lake City, UT 84107, USA; Benjamin.Horne@imail.org
4Division of Cardiovascular Medicine, Department of Medicine, Stanford University,
Stanford, CA 94063, USA
5Department of Surgery, University of Utah School of Medicine, Salt Lake City, UT 84132, USA
6Department of Pediatrics, University of Utah School of Medicine, Salt Lake City, UT 84132, USA
7Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT 84112, USA;
8Department of Economics, Brigham Young University, Provo, UT 84602, USA;
email@example.com (N.C.C.); firstname.lastname@example.org (C.A.P.III)
Received: 31 August 2020; Accepted: 6 October 2020; Published: 13 October 2020
Utah’s low-smoking population and high population density concentrated in mountain
valleys, with intermittent industrial activity and frequent temperature inversions, have yielded
unique opportunities to study air pollution. These studies have contributed to the understanding
of the human health impacts of air pollution. The populated mountain valleys of Utah experience
considerable variability in concentrations of ambient air pollution because of local emission sources
that change over time and episodic atmospheric conditions that result in elevated concentrations of
air pollution. Evidence from Utah studies indicates that air pollution, especially combustion-related
ﬁne particulate matter air pollution and ozone, contributes to various adverse health outcomes,
including respiratory and cardiovascular morbidity and mortality and increased risk of lung cancer.
The evidence suggests that air pollution may also contribute to risk of pre-term birth, pregnancy loss,
school absences, and other adverse health outcomes.
Keywords: air pollution; mortality and morbidity; Utah; particulate matter; ozone
The state of Utah has a long history of poor air quality that has provided substantial opportunities
to study the health eﬀects of air pollution. Most of Utah’s land mass includes sparsely populated
mountains, deserts, and canyonlands with relatively clean, less polluted air. However, most Utahans
live in urban areas located in valley basins surrounded by mountains. Approximately 80% of the
population of Utah lives in a contiguous urban/suburban area called the Wasatch Front [
]. The Wasatch
Front is a relatively narrow landmass—approximately 80 miles (130 km) long from north to south and
5 to 18 miles (8–29 km) wide—bordered on the east by the Wasatch Mountain Range. The relatively
dense population of the Wasatch Front (compared to the rest of the state and most other areas in the
Intermountain West) and related industrial, traﬃc, and other emission sources result in substantial
levels of air pollution that can at times be the worst in the U.S. [
]. Furthermore, surrounding mountain
Atmosphere 2020,11, 1094; doi:10.3390/atmos11101094 www.mdpi.com/journal/atmosphere
Atmosphere 2020,11, 1094 2 of 14
topography and interaction with meteorological conditions lead to considerable temporal variability
in air pollution.
Changes in the operation of major industrial sources of air pollution in Utah—including the
intermittent operation of a steel mill and copper smelter—have provided unique natural experiments
or quasi-experimental conditions to explore health impacts of air pollution. Utah also consistently
holds the nation’s lowest smoking rate (9% compared to the national average of 17.1%) [
], which helps
evaluate the health eﬀects of air pollution with less potential for confounding from smoking.
Studies of particulate matter air pollution have been the primary focus in Utah. Monitoring of
m in aerodynamic diameter) began at Utah monitoring sites in the mid to late
1980s, and regular monitoring of PM
m in aerodynamic diameter) began at some
Utah monitoring sites in 1999. A growing number of studies have also investigated the eﬀects of ozone
) and nitrogen oxide (NO
) pollutants. These pollutants are growing in relevance because Utah’s
four largest counties (Salt Lake, Utah, Davis, and Weber, which account for 75% of the population)
are currently not compliant with federal national ambient air quality standards (NAAQS) for ozone.
This problem is projected to worsen as emissions increase from a rapidly growing population and
climate change threatens to increase ground-level ozone production [
]. Since 2009, Utah’s population
grew 14 percent to a current total of
3 million persons [
]. The population is anticipated to grow
to 5.8 million in 2065, which represents a rate of change of 1.3 percent. This is nearly double the
0.7 percent growth seen nationally from 2016 to 2017 [
]. Utah has reduced air emissions by 38 percent
during the past 15 years, but the winter temperature inversions still pose a major problem for the state,
and summertime ozone emissions are emerging as a major public health concerns .
Early epidemiology studies of air pollution in Utah contributed to the 1971 U.S. Environmental
Protection Agency (EPA) sponsored Community Health and Environmental Surveillance System
(CHESS) studies. The CHESS studies in Utah were focused on the health eﬀects of particulate matter
and sulfur oxides, with a speciﬁc focus on sulfur dioxide and sulfates [
]. There was substantial
controversy surrounding the CHESS studies, compromising the ability for the program to change
public policy and move air pollution science forward.
Air pollution research in Utah resumed in the mid to late 1980s, including a unique natural
experiment related to the intermittent operation of a major industrial source of pollution [
]; a series
of panel studies on the link between air pollution, pulmonary function, and respiratory illness [
and early time-series studies of daily mortality [
]. These Utah-based studies were included in
the Environmental Protection Agency’s 1996 Air Quality Criteria for Particulate Matter Final Report,
which laid the groundwork for ﬁne particulate matter (PM
) standards [
]. Today, Utah-based
studies remain a part of a much larger and broader body of research that explores the health eﬀects of
air pollution. This article brieﬂy summarizes the results of existing Utah-based studies of pollution’s
eﬀects on multiple health outcomes including mortality, respiratory disease, cardiovascular disease,
cancer survivorship, and birth outcomes (Table 1).
Atmosphere 2020,11, 1094 3 of 14
Table 1. Outline and review of air pollution and health studies from Utah.
Studies Health End Points Study Designs Result Summary of Results
Mortality, all Cause, Cardiovascular, Respiratory
Mortality associated with higher levels of air
pollution, especially combustion/industrial-source
ﬁne particles versus windblown ﬁne particles.
Archer 1990  Respiratory and lung cancer mortality 3-county ecologic +
Pope et al., 1992,1996,1999 [14,15,18] All, cardiovascular, respiratory mortality Daily time series +
Lyon et al., 1995  All, cardiovascular, respiratory mortality Daily time series 0
Styer et al., 1995  All mortality Daily time series 0
Ransom et al., 1995  All mortality and hospitalization Natural experiment +
Pope et al., 2007  All mortality Natural experiment +
Epidemiological and toxicological evidence that
air pollution contributes to pulmonary
inﬂammation, respiratory illness and disease and
reduced pulmonary function.
Love et al., 1982  Respiratory illness incidence and severity Panel 0
Lutz 1983  Diagnosis of respiratory or cardiac illness Episode study +
Pope 1989, 1991 [10,11] Hospitalization for respiratory illness Natural experiment +
Pope et al., 1991,1992 [12,13] Respiratory symptoms/function in children Panel/time series +
Pope and Kanner 1993 
Pulmonary function eﬀects for former smokers
Ghio et al., 2001  Inﬂammatory lung injury in humans Human in vivo +
Dye et al., 2001  Pulmonary toxicity in rats Toxicology +
Watterson et al., 2007  Gene expression Toxicology +
Beard et al., 2012  Emergency Department visits for asthma Case-crossover +
Pirozzi et al., 2015a, 2015b [29,30] Respiratory eﬀects for former smokers Observational/panel +/0
Horne et al., 2018  Respiratory infection Case crossover +
Pirozzi et al., 2018  Pneumonia incidence Case crossover +
Wagner et al., 2018, 2020 [33,34] Aerobic, pulmonary function Human experiment 0
Evidence that air pollution contributes to risk of
various cardiovascular events, impairs cardiac
autonomic function, and contributes to
inﬂammation and endothelial injury.
Pope et al., 1999b, 1999c, 2004 [35–37] Heart rate variability Panel/human experiment +/0
Pope et al., 2006, 2015 [38,39] Ischemic heart disease events Case crossover +
Pope et al., 2008  Heart failure, hospitalization Case crossover +
O’Toole et al., 2010  Endothelial progenitor cells Panel/human experiment +
Bunch et al., 2011  Atrial ﬁbrillation hospitalization Case crossover 0
Pope et al., 2016  Endothelial injury and inﬂammation Panel/human experiment +
Leiser et al., 2019  Cardiac hospital readmission/death Medicare cohort +
Atmosphere 2020,11, 1094 4 of 14
Table 1. Cont.
Studies Health End Points Study Designs Result Summary of Results
Evidence that radioactive fallout is associated with
cancer mortality. Contributes to evidence that air
pollution may be associated with cancer and
Lyon et al., 1981  Lung cancer incidence Case–control 0
Archer 1990  Lung cancer mortality 3-county ecologic +
Blindauer et al., 1993  Lung cancer incidence Natural experiment 0
Ball et al., 2008  Lung, kidney, non-Hodgkin’s Ecological +
Ou et al., 2019  Respiratory health in cancer survivors Case crossover +
Ou et al., 2020  Various types of cancer mortality Cohort +
Contributes to evidence of an association between
air pollution and pre-term birth.
Parker et al., 2008  Pre-term birth Natural experiment +
Mendola et al., 2019  Pre-term birth Observational cohort +
Leiser et al., 2019  Pregnancy loss Case crossover 0
Evidence that air pollution is associated with an
increase in school absences and other health
outcomes. Evidence for higher exposure amongst
Ransom et al., 1992  School absences Daily time series +
Zeft et al., 2009  Juvenile idiopathic arthritis Case crossover +
Bakian et al., 2015  Suicide Case crossover +
Hales et al., 2016  School absences Natural experiment +
Youngquist et al., 2016  Emergency Medical Service calls Case crossover 0
Mullen et al., 2019  Racial/ethnic exposure disparity Exposure modeling +
Collins et al., 2019  Racial/ethnic exposure disparity Cross-sectional +
+ = positive association; 0 =no association; −=inverse association.
Atmosphere 2020,11, 1094 5 of 14
Utah-based studies provide evidence of an association between air pollution and mortality.
Archer evaluated longitudinal diﬀerences in mortality across three counties, contrasting death rates
during periods of intermittent operation of a steel mill that was constructed during World War II in one
of the counties (Utah County) [
]. This initial analysis was based on a simple ecological design that
compared mortality in Utah County to two other study areas without a similar source of industrial
pollution. It was estimated that 30 to 40% of respiratory cancer and nonmalignant respiratory disease
deaths in one of these areas were associated with community air pollution emitted from the steel
]. Additionally, analyses that treated the intermittent operations of a local steel mill [
the intermittent operation copper smelter [
] as natural experiments further observed that mortality
was associated with ﬁne-combustion and industrial-source particulate air pollution.
Several population-based, daily time-series studies that evaluated day-to-day changes in mortality
counts with short-term (1–5 days) changes in air pollution have been conducted using Utah’s Wasatch
Front counties (Salt Lake, Utah, Davis, and Weber). The earliest study reported a 16% increase in
mortality counts in Utah County associated with a 100
increased in exposure to particulate
matter air pollution, measured as PM
over the previous 5 days, after controlling for time trends,
seasonality, temperature, and relative humidity [
]. Pollution was most strongly associated with
respiratory and cardiovascular deaths. Additional analyses of Utah County mortality data conﬁrmed
the PM–mortality association but questioned if the association was causal [
]. An extended analysis,
however, demonstrated similar PM–mortality associations of (between 11–16% per 100
using alternative synoptic weather modeling approaches to control for weather, suggesting that the
observed PM–mortality associations were not the results of confounding by weather variables .
A daily time-series analysis of mortality counts and air pollution in a neighboring Wasatch Front
county (Salt Lake County) did not ﬁnd evidence of an association between mortality and PM
A more comprehensive population-based daily time-series mortality study was conducted using the
populations from all three primary metropolitan areas of the Wasatch Front including the following:
the Ogden area (Weber County), the Salt Lake City area (Salt Lake and Davis Counties), and the
Provo/Orem area (Utah County) [
]. The Salt Lake City area experienced many more high PM episodes
dominated by windblown dust. When the pollution data were screened to exclude windblown dust
episodes (based on clearing index screening), comparable PM
–mortality associations were observed
across the Wasatch Front metropolitan areas (between 0.8–1.6% change in mortality per 10
It was concluded that stagnant air pollution episodes with higher concentrations of combustion-source
and industrial-source ﬁne particles were more strongly associated with elevated mortality (as opposed
to windblown dust episodes with higher coarse, crustal derived particles).
3. Lung Disease and Respiratory Health Outcomes
3.1. Human Health Outcomes
Multiple Utah-based studies reported that air pollution is associated with adverse respiratory
health outcomes. As noted above, population-based daily time-series mortality studies observe that
short-term increases in pollution are associated with increased respiratory mortality counts [
Short-term increases in PM
during winter inversions are associated with signiﬁcant
increases in the risk estimates for outpatient visits, emergency department visits, and hospital
admissions for respiratory disease in multiple counties [
]. These studies found that there
were nearly twice as many respiratory hospital admissions for children during the period the steel mill
was operating compared to when the steel mill was not operating [
]. One study found that for
months when PM
was over 50
, the average annual standard at the time, hospital admissions
increased by 89% for children and 47% for adults .
Respiratory infections are a particular concern as acute lower respiratory infections, bronchitis,
and pneumonia have signiﬁcant associations with short-term increases in particulate matter
Atmosphere 2020,11, 1094 6 of 14
]. Pediatric populations are particularly vulnerable to respiratory infections associated
with short-term increases in PM
. One study found that the odds ratio for acute lower respiratory
infection in young children (0–2 years of age) is 1.15 (95% CI: 1.12–1.19) per 10
, with a lag
period of up to 28 days [
]. The associations are somewhat larger for pneumonia, with odds ratios
ranging from 1.35–1.50 [
]. Pediatric populations exposed to high levels of PM
also report increased
use of asthma medication, coughing, and increases in reported symptoms of respiratory disease [
3.2. Lung Function and Performance
Utah-based studies provide evidence that air pollution is associated with reduced lung function
in susceptible populations. Studies of 16 healthy adults exposed to PM
below the federal 24 h health
standard found no negative eﬀects on respiratory function or aerobic performance after 20 min of heavy
], but multiple studies reported signiﬁcant eﬀects on lung function among children and
adults with preexisting chronic obstructive pulmonary disease (COPD) [
]. Among fourth and
ﬁfth grade elementary students, 150
increases in PM
were associated with a 3–6% decline in
lung function [
]. In a cohort of ﬁfth and sixth graders, short-term increases in PM
with declines in peak expiratory ﬂow (PEF), irrespective of exhibited symptoms [
]. Among adult
smokers with COPD, an increase of 100
was signiﬁcantly associated with a 2% decrease
in forced expiratory volume [
]. For adult COPD patients, respiratory symptoms signiﬁcantly
increased after days with increased PM2.5 .
Multiple laboratory and human biomarker studies support local inﬂammation as the primary
mechanism by which particulate matter and ozone pollution exert adverse eﬀects on the respiratory
study reported that human bronchial epithelial cells (BEAS-2B) exposed to PM
found in Cache Valley signiﬁcantly upregulated genes activating receptors to interleukins 1 and
6 (IL-1R1 and IL-6R), IL-6 and phosphorylated STAT3 protein release, indicating activation of the
IL-6/gp130/STAT3 signaling pathway [
]. The study also reported slight cytotoxicity of the Cache
study examined the eﬀect of PM
particulates collected during the
period of intermittent steel mill operation installed in the trachea of Sprague–Dawley rats. Rats exposed
collected during steel mill operation expressed signiﬁcant pulmonary injury and neutrophilic
inﬂammation, which was suggested to be due to metals contained in the particulate matter .
study installed aqueous extracts of PM collected during intermittent steel
mill operation over a 3-year period inside the lungs of 24 nonsmoking healthy volunteers.
Subjects administered the extracts of PM from ﬁlters taken while the steel mill was in operation
had signiﬁcantly high levels of neutrophil inﬁltration and elevated concentrations of ﬁbronectin and
-1-antitrypsin, indicating inﬂammatory lung injury [
]. In a separate study, human exhaled breath
condensate was collected from former smokers with moderate to severe COPD on days with PM
that was considered “clean” and on days with higher PM
pollution during winter inversions [
levels were associated with increases in nitrite plus nitrate (NO
), a biomarker of oxidative
stress in COPD patients (mean of 3.16 diﬀerence between polluted and clean days), but not former
smokers without COPD. Ozone was also examined as a potential pulmonary inﬂammatory agent
among individuals with COPD. High ozone was associated with increased NOx and thus oxidative
stress and pulmonary inﬂammation in both COPD patients (8.7 vs. 28.6 on clean versus polluted days)
and persons without COPD (7.6 vs. 28.5), with no diﬀerence between the groups .
4. Cardiovascular Disease
Several Utah-based studies have found an association between air pollution and cardiovascular
health. As noted above, population-based daily time-series mortality studies have observed that short-term
increases in pollution are associated with increased cardiovascular mortality [
studies of patients drawn from a large cardiac catheterization registry who lived in the Wasatch Front
Atmosphere 2020,11, 1094 7 of 14
area of Utah observed that a 10
increase in PM
air pollution was associated with a 4.5% (95% CI:
1.1–8.0) increased risk of acute ischemic coronary events (unstable angina and myocardial infarction) [
The elevated risk was primarily observed among patients with angiographically demonstrated underlying
coronary artery disease. An additional similar case-crossover study provided further evidence that,
for patients living on Utah’s Wasatch Front, a 10
increase in PM
exposures contribute to the
triggering of acute coronary events (OR 1.06, 95% CI: 1.02–1.11), especially ST-segment elevation myocardial
infarction (OR 1.15, 95% CI: 1.03–1.29) [
]. Case-crossover studies of hospitalizations further observed
that air pollution was associated with heart failure hospitalizations (13.1% increase per 10 µg/m3, 95% CI:
] but not with hospitalizations for atrial fibrillations [
]. A cohort study examining the risk of
hospital readmission and death after cardiovascular events found that an increase of 10
to a 25–30% increased risk of readmission .
Utah studies have explored pathophysiological pathways that link exposure to air pollution
with cardiovascular disease. PM
air pollution in Utah has been associated with changes
in cardiac autonomic function as measured by measures of heart rate variability [
blood markers of inﬂammation [
], decreasing circulating levels of endothelial progenitor cells [
and vascular/endothelial injury [
]. A recent study conducted in Utah County analyzed blood drawn
from panels of healthy, nonsmoking young adults. The timing of multiple blood draws took advantage
of frequent persistent temperature inversion episodes, allowing for blood draws at times with varying
levels of exposure to PM
]. Increased air pollution exposure was associated with elevated
immune cells, a systemic increase in inﬂammatory and antiangiogenic cytokines with suppression
of proangiogenic growth factors. Additionally, elevated air pollution exposure was associated with
increased circulating endothelial microparticles, indicating endothelial cell apoptosis and vascular
Although air pollution is currently classiﬁed by the International Agency for Research on Cancer as
a known carcinogen and extensive research support its association with incident cancer (especially lung
cancer) and cancer mortality [
], published studies in Utah on the topic are limited and inconclusive.
A cluster investigation around point sources of pollution reported no signiﬁcant increase in the
incidence of cancers among residents near a coke oven in a steel mill, but did report a slight increase
in the number of excess lung cancers near the coke ovens [
]. In a separate study, mortality from
respiratory cancers in a low-smoking county with a steel mill was estimated to be 38% higher than
mortality in a neighboring low-smoking county without a steel mill [
]. In a case–control study that
adjusted for smoking, no consistent diﬀerence was found in the rates of lung cancer incidence between
Utah county and several other counties [
]. A cluster study of lung, kidney, and non-Hodgkin
lymphomas found elevations in the number of kidney (Risk Ratio (RR) range: 0.50–3.17) and lung
cancers (RR range: 1.02–1.51) around Hill Air Force Base, with the authors attributing the elevated risk
of cancers to water contamination from the Air Force base rather than potential air pollutants from the
base or other emission sources .
To date, most research on the topic of air pollutants and cancer focused on cancer incidence or
mortality in a general population. Researchers in Utah were the ﬁrst to investigate the eﬀect of air
pollution on the pulmonary health and mortality of cancer survivors after diagnosis. Cancer itself and
the long-term toxic eﬀects of cancer therapies on cancer survivors may increase their susceptibility
to health events and mortality associated with air pollution. In a statewide case-crossover study of
childhood cancer survivors, PM
was associated with signiﬁcant increases in the risk for respiratory
hospitalization and emergency room visits or hospitalization for respiratory infection. The risk
for respiratory events was signiﬁcantly higher among childhood cancer survivors than population
comparisons without a cancer history (OR 1.84, 95% CI: 1.13–3.00 per 10
from diagnosis through 5 and 10 years after diagnosis was also associated with all cause and cancer
mortality among pediatric, adolescent, and young adult survivors (AYA) with certain diagnoses [
Atmosphere 2020,11, 1094 8 of 14
Pediatric patients diagnosed at age 14 years or younger with lymphoma (1.34, 95% CI: 1.06–1.68) or
central nervous system (CNS) tumors (1.27, 95% CI: 1.05–1.52) had a signiﬁcant increase in their risk for
cancer mortality associated with a 5
increase in PM
exposure within 10 years from diagnosis.
Among AYAs diagnosed from age 15 to 39 years, PM
was associated with all cause and cancer
mortality among survivors with central nervous system tumors (1.20, 95% CI: 1.04–1.38), breast cancer
(1.16, 95% CI: 0.97–1.39), and colorectal cancer (1.23, 95% CI: 1.00–52) within 10 years of diagnosis.
6. Birth Outcomes
Human epidemiologic studies in Utah of the association between air pollutant exposure and birth
outcomes are rare. Two studies reported signiﬁcant associations between increased exposure to PM
and risk of preterm births [
]. The sources of PM
and pollutants studies varied between the
studies. An earlier study implemented a quasi-experimental design to examine the eﬀect of a steel
mill closure on preterm births and birth weight [
]. Utah mothers who were pregnant around the
time of the steel mill closure were less likely to have a preterm birth than mothers who were pregnant
before or after the closure (RR 0.95, 95% CI: 0.77–1.18). Reducing exposure during the second trimester
appeared to have the highest eﬀects in reducing preterm births. No eﬀects on birth weight were
observed, but the authors acknowledged the small sample size found in the study and their lack of
exact exposure estimates for each mother.
The latter study examined the association of preterm births and air pollution while accounting for
the composition of the pollution among mothers with two consecutive pregnancies [
]. This study
reported that high exposure to sulfur dioxides, ozone, nitrogen oxides, carbon monoxides, and particles
had a positive association with second pregnancy preterm births (range of 17–43% increase
in risk). Only one Utah-based study examined the associations of PM
and nitrogen dioxide with
spontaneous pregnancy loss [
]. The authors reported a signiﬁcant 16% increase in the odds of
spontaneous pregnancy loss associated with a 10 ppb increase in 7-day levels of nitrogen dioxide,
and positive but non-signiﬁcant associations with 3- and 7-day averages of PM2.5 .
7. Other Outcomes
Air pollution appears has additional wide-ranging eﬀects on society that may aﬀect public use of
emergency services, education, and mental health. Utah-based studies report signiﬁcant associations
between short-term PM
exposure and emergency services calls for diabetic symptoms, but no
signiﬁcant associations with CV or respiratory symptoms [
]. School absences may also be associated
, with eﬀects varying by the lag periods of interest and scope of the study.
A single-county study found a signiﬁcant association between a 28-day moving average of PM
equal to 100
with a 2% increase in the rate of school absences in one school district and an
elementary school in Utah county [
]. A study of school districts in multiple Utah counties recently
reported a similar ﬁnding that a 10
increase in PM
was associated with a 1.7% increase in daily
elementary school absences. These ﬁndings were robust even after controlling for structural factors
such as seasonal trends across school years, day-of-week eﬀects, holiday eﬀects, and weather .
An emerging body of research reported signiﬁcant associations between suicide completion and
increases in speciﬁc air pollutants. In Utah, interquartile-range increases in 3-day cumulative averages
of nitrogen dioxide and lag-day increases in PM
were associated with an increased suicide risk
(OR 1.20, 95% CI: 1.04–1.39 and OR 1.05, 95% CI: 1.01,1.10, respectively). Exposure to nitrogen dioxide
during the spring and fall and exposure to PM
during the spring were reported as having signiﬁcant
associations with suicide .
Although air pollution has systemic inﬂammatory eﬀects, few studies in Utah have examined
its association with arthritic disease. A single study of preschool aged children living in Utah’s
Wasatch Front reported signiﬁcant associations between 14-day increases in PM
a signiﬁcant elevation in the risk of juvenile idiopathic arthritis (JIA) (RR 1.60, 95% CI: 1.00–2.54).
The risk was higher in males than females and in patients with systemic onset JIA .
Atmosphere 2020,11, 1094 9 of 14
Inequities in exposure to air pollutants by racial and ethnic groups have also been observed in
]. A study of air pollution exposure in Salt Lake City estimated that schools with higher
proportions of racial/ethnic minority students were consistently exposed to more PM
(Hispanics and RRs: 1.02–1.12; non-Hispanic minorities and RRs: 1.01–1.04) [
]. Another study of Salt
Lake City residents reported inequalities in air pollution exposure across diﬀerences in race, ethnicity,
and religion (negative association between PM
concentration and White percentage, p
8. Summary and Conclusions
Utah-based air pollution studies have made important contributions to understanding the health
eﬀects of air pollution. As would be expected, the evidence suggests that Utahans experience similar
health eﬀects of air pollution as observed elsewhere [
]. Figure 1summarizes key health eﬀects
of air pollution based on evidence from the overall scientiﬁc literature, with the eﬀects that have
been observed in Utah-based studies highlighted [
]. Figure 2is a photograph of the pollution in
Utah Valley around the local steel mill, the health eﬀects of which have been examined in multiple
studies. The majority of research in Utah has been focused on particulate matter’s eﬀects on human
health outcomes. The particulate matter measures in past studies largely originated from industrial
emissions, wood burning, and mobile transportation sources. Although the steel mill featured in
several Utah-based studies closed in the early 2000s and Utah has seen improvements in air quality
since the 1990s, Utah has seen a regular annual increase in emissions of particulate matter pollution
from wildﬁres that are increasing in frequency and severity as a result of warmer and drier conditions
due to climate change [
]. Particulate matter from these wildﬁres originates from ﬁres in Utah and
in states across the west coast. The composition of the particulate matter from these wildﬁres likely
diﬀers from the prior particulate matter studied, the health eﬀects of which have yet to be determined.
Figure 1. Health effects of air pollution (attached). Adapted from Thurston et al. 2017  and The
Utah Roadmap 2020 .
Health eﬀects of air pollution (attached). Adapted from Thurston et al. 2017 [
] and The
Utah Roadmap 2020 .
Atmosphere 2020,11, 1094 10 of 14
Figure 1. Health effects of air pollution (attached). Adapted from Thurston et al. 2017  and The
Utah Roadmap 2020 .
Photograph of Geneva Steel, Utah Valley, 1989 (PM
m in aerodynamic
diameter) approximately 150 µg m−3).
Ozone and nitrogen oxides are air pollutants present in Utah, but their health eﬀects on the Utah
population are understudied relative to the research on particulate matter pollution. In addition,
Utah is one of the nation’s largest emitters of toxic air emissions [
], primarily due to a large copper
mine located west of the Wasatch Front. Few studies in Utah have examined how exposures to air
toxics inﬂuence health outcomes such as cancer incidence or cancer mortality. Future directions for air
pollution studies in Utah include more human health studies that examine associations between health
and wildﬁre smoke, ozone, nitrogen oxides, air toxics, and the results of multi-pollutant exposures.
Furthermore, as new low-cost and mobile air quality measurements are deployed, studies could focus
on the spatial pattern of health eﬀects at smaller spatial scales than counties.
Utah has experienced rapid population growth in recent decades largely along the already densely
populated Wasatch Front. The majority of Utahans utilize cars as their primary means of transportation.
Governmental policies that can increase the availability and use of public transportation and city zoning
laws that can reduce the number of residences located near mobile and point sources of pollution are not
uniformly implemented across the Wasatch Front. Consequently, air pollution emissions in Utah from
mobile sources and chronic exposure to mobile emissions may increase as the population continues to
grow unless public policy can accelerate the adoption of low- or zero-emission technologies .
Utah-based studies and many others across the world report a substantial number adverse health
eﬀects due to air pollution exposure. Despite decades of research, the debate about air pollution and
its health implications continues. One of the primary arguments against the ﬁndings of air pollution
studies is the role of causation in human epidemiologic studies. Opponents of air pollution policies
argue that human epidemiologic studies leave too much uncertainty around whether air pollution
truly has a casual eﬀect on the adverse health outcomes documented in the literature. Because air
pollution exposure is widespread, ﬁnding counterfactual populations to demonstrate causality in air
pollution studies can be challenging. Utah is home to the nation’s only population database, the Utah
Atmosphere 2020,11, 1094 11 of 14
Population Database (UPDB), which can create longitudinal residential histories for all Utahans from
ﬁrst residence or birth in the state to death or emigration from the state by linking driver license,
vital records, voter registration, and marriage and divorce records to personal identifying information.
Data about air pollution exposure at the address level, all medical history and cancer diagnoses for Utah
residents, vital status, and family history of disease for all persons living in the state are also available
through the UPDB. Future studies on the topic of health outcomes in Utah can address questions
regarding causality in epidemiologic studies by leveraging the long-term follow up, comprehensive
capture of confounders like smoking, longitudinal residential history, and matching capabilities of the
Utah Population Database.
Utah’s contribution to the literature on the health impacts of air pollution will continue to grow.
Future topics of relevance include studies of governmental policies addressing air pollution exposure;
inequities in these exposures; health studies that incorporate air toxics and ozone and nitrogen oxides;
understanding how air pollution contributes to COVID-19; eﬀects of wildﬁres exacerbated by climate
change; source apportionment studies and their associated health eﬀects; and sub-county gradients in
air pollution. The unique pollution patterns, data resources, and scientiﬁc capacity in Utah can be
leveraged to address knowledge gaps that remain in this ﬁeld.
Conceptualization: J.O., C.S.P., B.D.H., L.E.M., C.A.P.III; Methodology: J.O., C.S.P., B.D.H.,
L.E.M., N.C.C., C.A.P.III; Resources: J.O., B.D.H., C.A.P.III; Writing—original draft preparation: J.O., N.C.C.,
C.A.P.III; Writing—review and editing: J.O., C.S.P., B.D.H., H.A.H., A.C.K., L.E.M., N.C.C., C.A.P.III; Visualization:
J.O., C.S.P., B.D.H., N.C.C., C.A.P.III; Supervision: J.O., C.A.P.III; Project administration: J.O., N.C.C., C.A.P.III.
All authors have read and agreed to the published version of the manuscript.
This research received no external funding. C.A.P.III was funded in part by the Mary Lou Fulton
Professorship, Brigham Young University.
Claire Davis provided the original illustrations for the visual abstract (Figure 1) used in
Conﬂicts of Interest: The authors declare no conﬂict of interest.
Utah Population. 2020. Available online: https://worldpopulationreview.com/states/utah-population
(accessed on 23 August 2020).
American Lung Association State of the Air. 2019. Available online: http://www.stateoftheair.org/assets/sota-
2019-full.pdf (accessed on 23 August 2020).
Tobacco Use in Utah. 2019. Available online: https://truthinitiative.org/research-resources/smoking-region/
tobacco-use-utah2019 (accessed on 23 August 2020).
Orru, H.; Ebi, K.L.; Forsberg, B. The interplay of climate change and air pollution on health. Curr. Environ.
Health Rep. 2017,4, 504–513. [CrossRef] [PubMed]
Archer, C.L.; Brodie, J.F.; Rauscher, S.A. Global warming will aggravate ozone pollution in the U.S.
Mid-Atlantic. J. Appl. Meteorol. Clim. 2019,58, 1267–1278. [CrossRef]
Leins, C. Best States: Utah Faces Unique Air Quality Challenges. US News & World Report. 2019.
Available online: https://www.usnews.com/news/best-states/articles/2019-06-25/utah-works-to-address-its-
unique-air-quality-challenges (accessed on 23 August 2020).
Perlich, P.; Hollingshaus, M.; Harris, R.; Tennert, J.; Hogue, M. Utah’s Long-Term Demographic and Economic
Projections Summary; Kem, C. Gardner Policy Institute: Salt Lake City, UT, USA, 2017.
Community Health and Environmental Surveillance System; Finklea, J.F.; Goldberg, J.; Hasselblad, V.;
Shy, C.M.; Hayes, C.G. Health Consequences of Sulfur Oxides: A Report from CHESS 1970–1971;
National Environmental Research Center: Nagpur, India, 1974.
Love, G.J.; Lan, S.P.; Shy, C.M. A study of acute respiratory disease in families exposed to different levels of air
pollution in the Great Salt Lake basin, Utah, 1971–1972 and 1972–1973. Environ. Health Perspect.
Pope, C.A., III. Respiratory disease associated with community air pollution and a steel mill, Utah Valley.
Am. J. Public Health 1989,79, 623–628. [CrossRef] [PubMed]
Atmosphere 2020,11, 1094 12 of 14
Pope, C.A., III. Respiratory Hospital Admissions Associated with PM10Pollution in Utah, Salt Lake, and Cache
Valleys. Arch. Environ. Health Int. J. 1991,46, 90–97. [CrossRef]
Pope, C.A., III; Dockery, D.W.; Spengler, J.D.; Raizenne, M.E. Respiratory health and PM10Pollution: A daily
time series analysis. Am. Rev. Respir. Dis. 1991,144, 668–674. [CrossRef] [PubMed]
Pope, C.A., III; Dockery, D.W. Acute health eﬀects of PM10Pollution on symptomatic and asymptomatic
children. Am. Rev. Respir. Dis. 1992,145, 1123–1128. [CrossRef] [PubMed]
Pope, C.A., III; Schwartz, J.; Ransom, M.R. Daily mortality and PM 10 Pollution in Utah Valley. Arch. Environ.
Health Int. J. 1992,47, 211–217. [CrossRef]
Pope, C.A., III; Kalkstein, L.S. Synoptic weather modeling and estimates of the exposure-response relationship
between daily mortality and particulate air pollution. Environ. Health Perspect.
,104, 414–420. [CrossRef]
US Environmental Protection Agency. Air Quality Criteria for Particulate Matter (Final Report, April 1996);
US Environmental Protection Agency: Washington, D.C., USA, 1996; Volume 3.
Archer, V.E. Air pollution and fatal lung disease in three Utah counties. Arch. Environ. Health Int. J.
1990,45, 325–334. [CrossRef]
Pope, C.A., III; Hill, R.W.; Villegas, G.M. Particulate air pollution and daily mortality on Utah’s Wasatch
Front. Environ. Health Perspect. 1999,107, 567–573. [CrossRef] [PubMed]
Lyon, J.L.; Mori, M.; Gao, R. Is there a causal association between excess mortality and exposure to PM-10 air
pollution? Additional analyses by location, year, season, and cause of death. Inhal. Toxicol.
Styer, P.; McMillan, N.; Gao, F.; Davis, J.; Sacks, J. Eﬀect of outdoor airborne particulate matter on daily death
counts. Environ. Health Perspect. 1995,103, 490–497. [CrossRef] [PubMed]
Ransom, M.R.; Pope, C.A., III. External health costs of a steel mill. Contemp. Econ. Policy
Pope, C.A., III; Rodermund, D.L.; Gee, M.M. Mortality eﬀects of a copper smelter strike and reduced ambient
sulfate particulate matter air pollution. Environ. Health Perspect. 2007,115, 679–683. [CrossRef]
23. Lutz, L.J. Health eﬀects of air pollution measured by outpatient visits. J. Fam. Pr. 1983,16, 307–313.
Pope, C.A., III; Kanner, R.E. Acute eﬀects of PM10Pollution on pulmonary function of smokers with mild to
moderate chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 1993,147, 1336–1340. [CrossRef]
Ghio, A.J.; Devlin, R.B. Inﬂammatory lung injury after bronchial instillation of air pollution particles. Am. J.
Respir. Crit. Care Med. 2001,164, 704–708. [CrossRef]
Dye, J.A.; Lehmann, J.R.; McGee, J.K.; Winsett, D.W.; Ledbetter, A.D.; Everitt, J.; Ghio, A.J.; Costa, D.L.
Acute pulmonary toxicity of particulate matter ﬁlter extracts in rats: Coherence with epidemiologic studies
in Utah Valley residents. Environ. Health Perspect. 2001,109, 395–403. [CrossRef]
Watterson, T.L.; Sorensen, J.; Martin, R.; Coulombe, R.A. Eﬀects of PM2.5 collected from Cache Valley Utah
on genes associated with the inﬂammatory response in human lung cells. J. Toxicol. Environ. Health Part A
2007,70, 1731–1744. [CrossRef]
Beard, J.D.; Beck, C.; Graham, R.; Packham, S.C.; Traphagan, M.; Giles, R.T.; Morgan, J.G. Winter
temperature inversions and emergency department visits for asthma in Salt Lake county, Utah, 2003–2008.
Environ. Health Perspect. 2012,120, 1385–1390. [CrossRef] [PubMed]
Pirozzi, C.S.; Sturrock, A.; Carey, P.; Whipple, S.; Haymond, H.; Baker, J.; Weng, H.-Y.; Greene, T.;
Scholand, M.B.; Kanner, R.; et al. Respiratory eﬀects of particulate air pollution episodes in former smokers
with and without chronic obstructive pulmonary disease: A panel study. COPD Res. Pr.
Pirozzi, C.S.; Sturrock, A.; Weng, H.Y.; Greene, T.; Scholand, M.B.; Kanner, R.; Iii, R.P.; Paine, R. Eﬀect of
naturally occurring ozone air pollution episodes on pulmonary oxidative stress and inﬂammation. Int. J.
Environ. Res. Public Health 2015,12, 5061–5075. [CrossRef] [PubMed]
Horne, B.D.; Joy, E.A.; Hofmann, M.G.; Gesteland, P.H.; Cannon, J.B.; Leﬂer, J.S.; Blagev, D.P.; Korgenski, E.K.;
Torosyan, N.; Hansen, G.I.; et al. Short-term elevation of ﬁne particulate matter air pollution and acute lower
respiratory infection. Am. J. Respir. Crit. Care Med. 2018,198, 759–766. [CrossRef]
Pirozzi, C.S.; Jones, B.; Vanderslice, J.A.; Zhang, Y.; Paine, R.; Dean, N.C. Short-term air pollution and incident
pneumonia. A case–crossover study. Ann. Am. Thorac. Soc. 2018,15, 449–459. [CrossRef]
Wagner, D.R.; Clark, N.W. Eﬀects of ambient particulate matter on aerobic exercise performance.
J. Exerc. Sci. Fit. 2018,16, 12–15. [CrossRef]
Atmosphere 2020,11, 1094 13 of 14
Wagner, D.R.; Brandley, D.C. Exercise in thermal inversions: PM2.5 air pollution eﬀects on pulmonary
function and aerobic performance. Wilderness Environ. Med. 2020,31, 16–22. [CrossRef]
Pope, C.A., III; Dockery, D.W.; Kanner, R.E.; Villegas, G.M.; Schwartz, J. Oxygen saturation, pulse rate,
and particulate air pollution. Am. J. Respir. Crit. Care Med. 1999,159, 365–372. [CrossRef]
Pope, C.A., III; Verrier, R.L.; Lovett, E.G.; Larson, A.C.; Raizenne, M.E.; Kanner, R.E.; Schwartz, J.; Villegas, G.;
Gold, D.R.; Dockery, D.W. Heart rate variability associated with particulate air pollution. Am. Hear. J.
1999,138, 890–899. [CrossRef]
Pope, C.A., III; Hansen, M.L.; Long, R.W.; Nielsen, K.R.; Eatough, N.L.; Wilson, W.E.; Eatough, D.J.
Ambient particulate air pollution, heart rate variability, and blood markers of inﬂammation in a panel of
elderly subjects. Environ. Health Perspect. 2004,112, 339–345. [CrossRef]
Pope, C.A., III; Muhlestein, J.B.; May, H.T.; Renlund, D.G.; Anderson, J.L.; Horne, B.D. Ischemic heart disease
events triggered by short-term exposure to ﬁne particulate air pollution. Circulation
Pope, C.A., III; Muhlestein, J.B.; Anderson, J.L.; Cannon, J.B.; Hales, N.M.; Meredith, K.G.; Le, V.; Horne, B.D.
Short-term exposure to ﬁne particulate matter air pollution is preferentially associated with the risk of
ST-segment elevation acute coronary events. J. Am. Hear. Assoc. 2015,4, e002506. [CrossRef] [PubMed]
Pope, C.A., III; Renlund, D.G.; Kfoury, A.G.; May, H.T.; Horne, B.D. Relation of heart failure hospitalization
to exposure to ﬁne particulate air pollution. Am. J. Cardiol. 2008,102, 1230–1234. [CrossRef] [PubMed]
O’Toole, T.E.; Hellmann, J.; Wheat, L.; Haberzettl, P.; Lee, J.; Conklin, D.J.; Bhatnagar, A.; Pope, C.A., III.
Episodic exposure to ﬁne particulate air pollution decreases circulating levels of endothelial progenitor cells.
Circ. Res. 2010,107, 200–203. [CrossRef]
Bunch, T.J.; Horne, B.D.; Asirvatham, S.J.; Day, J.D.; Crandall, B.G.; Weiss, J.P.; Osborn, J.S.; Anderson, J.L.;
Muhlestein, J.B.; Lappe, D.L.; et al. Atrial ﬁbrillation hospitalization is not increased with short-term
elevations in exposure to ﬁne particulate air pollution. Pacing Clin. Electrophysiol.
Pope, C.A., III; Bhatnagar, A.; McCracken, J.P.; Abplanalp, W.; Conklin, D.J.; O’Toole, T. Exposure to
ﬁne particulate air pollution is associated with endothelial injury and systemic inﬂammation. Circ. Res.
2016,119, 1204–1214. [CrossRef]
Leiser, C.L.; Smith, K.R.; Vanderslice, J.A.; Glotzbach, J.P.; Farrell, T.W.; Hanson, H.A. Evaluation of the
sex-and-age-speciﬁc eﬀects of PM2.5 on hospital readmission in the presence of the competing risk of
mortality in the medicare population of Utah 1999–2009. J. Clin. Med. 2019,8, 2114. [CrossRef]
Lyon, J.L.; Klauber, M.R.; Graﬀ, W.; Chiu, G. Cancer clustering around point sources of pollution:
Assessment by a case-control methodology. Environ. Res. 1981,25, 29–34. [CrossRef]
Blindauer, K.M.; Erickson, L.; McElwee, N.; Sorenson, G.; Gren, L.H.; Lyon, J.L. Age and smoking-adjusted
lung cancer incidence in a Utah County with a Steel Mill. Arch. Environ. Health Int. J.
Ball, W.; Lefevre, S.; Jarup, L.; Beale, L. Comparison of diﬀerent methods for spatial analysis of cancer data in
Utah. Environ. Health Perspect. 2008,116, 1120–1124. [CrossRef]
Ou, J.Y.; Hanson, H.A.; Ramsay, J.M.; Leiser, C.L.; Zhang, Y.; Vanderslice, J.A.; Pope, C.A., III; Kirchhoﬀ, A.C.
Fine particulate matter and respiratory healthcare encounters among survivors of childhood cancers. Int. J.
Environ. Res. Public Health 2019,16, 1081. [CrossRef] [PubMed]
Ou, J.Y.; Hanson, H.A.; Ramsay, J.M.; Kaddas, H.K.; Pope, C.A., III; Leiser, C.L.; Vanderslice, J.; Kirchhoﬀ, A.C.
Fine particulate matter air pollution and mortality among pediatric, adolescent, and young adult cancer
patients. Cancer Epidemiology Biomarkers Prev. 2020. [CrossRef] [PubMed]
Parker, J.D.; Mendola, P.; Woodruﬀ, T.J. Preterm birth after the Utah Valley Steel Mill closure. Epidemiology
2008,19, 820–823. [CrossRef] [PubMed]
Mendola, P.; Nobles, C.; Williams, A.D.; Sherman, S.; Kanner, J.; Seeni, I.; Grantz, K.L. Air pollution and
preterm birth: Do air pollution changes over time inﬂuence risk in consecutive pregnancies among low-risk
women? Int. J. Environ. Res. Public Health 2019,16, 3365. [CrossRef]
52. Leiser, C.L.; Hanson, H.A.; Sawyer, K.; Steenblik, J.; Al-Dulaimi, R.; Madsen, T.; Gibbins, K.; Hotaling, J.M.;
Ibrahim, Y.; Vanderslice, J.A.; et al. Acute eﬀects of air pollutants on spontaneous pregnancy loss:
A case-crossover study. Fertil. Steril. 2019,111, 341–347. [CrossRef]
Atmosphere 2020,11, 1094 14 of 14
Ransom, M.R.; Pope, C.A., III. Elementary school absences and PM10 pollution in Utah Valley. Environ. Res.
1992,58, 204–219. [CrossRef]
Zeft, A.S.; Prahalad, S.; Lefevre, S.; Cliﬀord, B.; McNally, B.; Bohnsack, J.F.; Pope, C.A., III. Juvenile idiopathic
arthritis and exposure to ﬁne particulate air pollution. Clin. Exp. Rheumatol. 2009,27.
Bakian, A.V.; Huber, R.S.; Coon, H.; Gray, D.; Wilson, P.; McMahon, W.M.; Renshaw, P.F. Acute air pollution
exposure and risk of suicide completion. Am. J. Epidemiology 2015,181, 295–303. [CrossRef] [PubMed]
Hales, N.M.; Barton, C.C.; Ransom, M.R.; Allen, R.T.; Pope, C.A., III. A Quasi-experimental analysis of
elementary school absences and ﬁne particulate air pollution. Med. 2016,95, e2916. [CrossRef]
Youngquist, S.T.; Hood, C.H.; Hales, N.M.; Barton, C.C.; Madsen, T.E.; Pope, C.A., III. Association between
EMS calls and ﬁne particulate air pollution in Utah. Air Qual. Atmosphere Health
,9, 887–897. [CrossRef]
Mullen, C.; Grineski, S.; Collins, T.; Xing, W.; Whitaker, R.; Sayahi, T.; Becnel, T.; Goﬃn, P.; Gaillardon, P.-E.;
Meyer, M.; et al. Patterns of distributive environmental inequity under diﬀerent PM2.5 air pollution scenarios
for Salt Lake County public schools. Environ. Res. 2020,186, 109543. [CrossRef] [PubMed]
Collins, T.W.; Grineski, S.E. Environmental injustice and religion: Outdoor air pollution disparities in
metropolitan Salt Lake City, Utah. Ann. Am. Assoc. Geogr. 2019,109, 1597–1617. [CrossRef]
Outdoor Air Pollution. Available online: https://monographs.iarc.fr/ENG/Monographs/vol109/mono109-F01.
pdf (accessed on 28 September 2020).
Brook, R.D.; Rajagopalan, S.; Pope, C.A., III; Brook, J.R.; Bhatnagar, A.; Diez-Roux, A.V.; Holguin, F.; Hong, Y.;
Luepker, R.V.; Mittleman, M.A.; et al. Particulate matter air pollution and cardiovascular disease. Circ.
2010,121, 2331–2378. [CrossRef]
Landrigan, P.J.; Fuller, R.; Acosta, N.J.R.; Adeyi, O.; Arnold, R.; Basu, N.; Bald
, A.B.; Bertollini, R.; Bose-O’Reilly, S.;
Boufford, J.I.; et al. The Lancet Commission on pollution and health. Lancet 2018,391, 462–512. [CrossRef]
Thurston, G.D.; Kipen, H.; Annesi-Maesano, I.; Balmes, J.; Brook, R.D.; Cromar, K.R.; De Matteis, S.;
Forastiere, F.; Forsberg, B.; Frampton, M.W.; et al. A joint ERS/ATS policy statement: What constitutes an
adverse health eﬀect of air pollution? An analytical framework. Eur. Respir. J.
The Utah Road Map. 2020. Available online: https://gardner.utah.edu/utahroadmap/(accessed on
10 August 2020).
Mallia, D.V.; Lin, J.C.; Urbanski, S.; Ehleringer, J.; Nehrkorn, T. Impacts of upwind wildﬁre emissions on CO,
CO2, and PM2.5 concentrations in Salt Lake City, Utah. J. Geophys. Res. Atmos.
,120, 147–166. [CrossRef]
Utah among the Most Toxic States, Report Says. Available online: https://www.deseret.com/2017/11/9/
20635999/utah-among-the-most-toxic-states-report-says (accessed on 10 August 2020).
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).