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https://doi.org/10.1038/s41477-018-0151-8
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
1Center for Public Health Research, Massey University, Wellington, New Zealand. 2USDA Forest Service, PNW Research Station, Portland, OR, USA.
3National Institute of Water and Atmospheric Research, Auckland, New Zealand. *e-mail: gdonovan@fs.fed.us
Since the pioneering work of Roger Ulrich1—who found that
patients with a view of a natural scene recovered more quickly
from surgery—research has shown that exposure to the natural
environment is associated with a wide range of beneficial health out-
comes, including lower mortality2,3, higher birth weight2–6, reduced
risk of cardiovascular disease7 and improved mental health8,9.
However, most studies have used a cross-sectional study design that
measured the natural environment and health outcomes at the same
point in time. This is a major limitation, as the health consequences
of environmental exposures often do not manifest immediately10,
and timing of exposure across the life course may strongly affect
the outcome, with the first few years of life believed to be particu-
larly critical11. In addition, past studies have relied heavily on a sin-
gle measure of overall greenness called the normalized difference
vegetation index (NDVI), which is typically derived from satellite
imagery. However, markedly different landscapes can have similar
NDVI values, so the use of NDVI has limited researchers’ capacity
to determine which aspects of the natural environment provide the
greatest public-health benefits.
Several recent studies suggest that exposure to the natural envi-
ronment may be protective of asthma, although the evidence is
mixed12. Asthma is a chronic health condition that affects 334 mil-
lion people worldwide and its prevalence is increasing13. Although
asthma treatment has improved, it is not effective for all asthmat-
ics, and there is no cure. In addition, there are limited public-health
interventions that can reduce asthma risk, and these interventions
are often difficult to implement. Therefore, there is considerable
interest in understanding how exposure to the natural environment
may protect against asthma and in identifying what specific aspects
of the natural environment afford the most protection.
An ecological study in 42 health catchment areas in New York
City14 showed that areas with more street trees within 1 square kilo-
metre had lower rates of asthma among 4–5 year olds, but there
was no relationship between tree density and hospitalizations for
asthma. A follow-up study15, using individual-level data from 549
African American and Dominican children, who lived in socially
deprived neighbourhoods in New York City, found that tree cover
was not protective of asthma. Furthermore, more trees within 250 m
of the prenatal address was a risk factor for allergic sensitization.
In a 10-year follow-up study of a large (n = 65,000) birth cohort
in Vancouver, British Columbia16, it was found that greenness was
protective of early-life asthma, but not of school-age asthma. The
authors measured greenness using NDVI. The results held after
controlling for air pollution and proximity to major roads. A study17
on greenness (measured using NDVI) and asthma rates in 14 met-
ropolitan statistical areas in Texas found no association, although
this may be an artefact of the ecological study design and small
sample size.
A study18 in 3,178 9–12 year olds in Sabadell, Spain, found no
relationship between greenness around a child’s residential address
(NDVI in buffers of 100 m to 1,000 m) and asthma. However, chil-
dren who lived closer to a park were at greater risk, which the
authors speculate may be because parks in Catalonia have more
non-native species than other green areas.
A recent cross-sectional study19 in Australia, not focused exclu-
sively on asthma, examined the relationship between the natural
environment and rates of hospital admission for respiratory disease
in local government areas. They found that multiple measures of the
natural environment were protective of a respiratory disease.
One reason for the observed inconsistent association between
the natural environment and asthma may be that NDVI is a rela-
tively coarse measure of greenness: different plants can have similar
spectral profiles. In addition, most studies only measured greenness
at one point in time and a single measure is unlikely to accurately
represent a child’s lifetime exposure. Indeed, using a one-point-in-
time measure may result in systematic measurement error as dem-
onstrated previously by Brokamp et al.20. The authors tracked all
the residential addresses (from birth to age 7) of a cohort of 613
children in Cincinnati, OH, and showed that, across time, children
moved to less socially deprived, greener neighbourhoods, with
Vegetation diversity protects against childhood
asthma: results from a large New Zealand birth
cohort
Geoffrey H. Donovan 1,2*, Demetrios Gatziolis2, Ian Longley3 and Jeroen Douwes1
We assessed the association between the natural environment and asthma in 49,956 New Zealand children born in 1998 and
followed up until 2016 using routinely collected data. Children who lived in greener areas, as measured by the normalized
difference vegetation index, were less likely to be asthmatic: a 1 s.d. increase in normalized difference vegetation index was
associated with a 6.0% (95% CI 1.9–9.9%) lower risk of asthma. Vegetation diversity was also protective: a 1 s.d. increase in
the number of natural land-cover types in a child’s residential meshblock was associated with a 6.7% (95% CI 1.5–11.5%) lower
risk. However, not all land-cover types were protective. A 1 s.d. increase in the area covered by gorse (Ulex europaeus) or exotic
conifers, both non-native, low-biodiversity land-cover types, was associated with a 3.2% (95% CI 0.0–6.0%) and 4.2% (95%
CI 0.9–7.5%) increased risk of asthma, respectively. The results suggest that exposure to greenness and vegetation diversity
may be protective of asthma.
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