Environmental control of outdoor allergens

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12 Current Allergy & Clinical Immunology | March 2019 | Vol 32, No 1
POLLENS
ollens are the male reproductive cells of most plants.1
Intact pollens range in size from 10–100 µm, but most have
diameters between 20–35 µm.2 Pollen counts are generally
lowest after rain and highest on warm, windy days.3 The
association between pollens and the onset of allergic disease
as well as exacerbations has been reported in several studies.4
Pollen allergy predominantly causes rhinoconjunctivitis because
the pollens are ltered mainly by the nasal mucosa and do not
reach the alveoli as a result of their large particle size. However,
pollen grains may rupture on contact with water, releasing starch
particles containing the allergens that are small enough to reach
the alveoli.2 Some grass allergens may bind to diesel-exhaust
carbon particles, leading to deposition in the airways.5 Even
though many asthmatics are sensitised to various pollen types,
pollen sensitisation has not been associated with an increased
risk of having asthma, although pollen exposure has been
associated with asthma exacerbations.6
MOULDS
Moulds require warmth, moisture and organic debris to grow,
with both the spores and the mycelia being allergenic.1 They are
potent triggers for asthma, as most fungal spores range in size
from 2 µm to 10 µm and they are therefore able to penetrate the
lower airway.1,7 Mould counts are usually higher in the evening
and rise after rain.1,3 Although mould is a perennial allergen,
there are seasonal spikes.1 Alternaria and Cladosporium species
are the major outdoor allergens responsible for sensitisation.8
Mould spores have been found to increase the risk of bronchial
hyper-responsiveness and asthma symptoms in children.6
Alternaria sensitivity has been found to be a risk factor for
severe asthma attacks and epidemic asthma.9
OUTDOOR AEROALLERGENS IN SOUTH AFRICA
In the Western Cape, the most abundant perennial aeroallergens
are fungal spores, the most common fungi being Alternaria,
Cladosporium and Epicoccum.10 The highest fungal spore
levels are recorded during spring and autumn.11 Grass pollens
are perennial allergens, but peak in October–December.10 Tree
pollens are seasonal allergens: oak pollen peaks in August,
plane tree in September and pine in October.10
On the Highveld, the grass-pollen season is from September to
May, with a mid-summer peak. The tree-pollen season is short
(October–November).11 In the Free State, grass and maize
pollen are the most common sensitising allergens in patients with
allergic rhinitis (AR) and asthma, with the rate of sensitisation to
tree pollen being much lower.12–14 Alternaria is the mould with
the highest rate of sensitisation in the Free State.15
In KwaZulu-Natal, grass pollen is the predominant outdoor
aeroallergen, with pollen levels peaking in spring and summer,
while fungal spores are a perennial allergen.11 In Mpumalanga,
grass-pollen levels peak in January, whereas fungal-spore
levels peak in February.11
Sensitisation to grass pollen and the development of hay fever
and/or seasonal conjunctivitis is more common in urban than
rural environments.16 Tree, weed and ower pollen are less
common as causes of allergy in South Africa as compared to
Europe.16
ABSTRACT
Outdoor allergens are an important cause of allergic rhinitis, conjunctivitis and asthma, with pollens and moulds being
the major outdoor allergens that induce symptoms in atopic patients. Grass pollens are the predominant pollen in South
Africa. Rising levels of atmospheric air pollution are linked to the exacerbation of allergic disease. Although pollens and
outdoor fungal spores are impossible to avoid completely, measures can be taken to reduce exposure in the outdoor
environment and reduce the amount of allergen entering indoors in order to improve disease control.
Key words: outdoor allergens, mould, pollen, aeroallergens, air pollution, allergic disease, sensitisation
Review Article
ENVIRONMENTAL CONTROL OF OUTDOOR
ALLERGENS
Riaz Y Seedat | MMed (ORL), FCORL (SA)
Department of Otorhinolaryngology, School of Clinical Medicine, Faculty of Health Sciences, University of the Free State,
Bloemfontein, South Africa
Email | seedatry@ufs.ac.za
P

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Current Allergy & Clinical Immunology | March 2019 | Vol 32, No 1
REVIEW ARTICLE
POLLUTION
Air pollution exposure is associated with increased rates of
allergic sensitisation and exacerbation of allergic disease.2,17
Exposure to ambient air pollutants in early childhood plays an
important role in the exacerbation of asthma and other respiratory
symptoms.4 Truck trac near residences is associated with
AR symptoms.18 Pollutants that can cause rhinitis and asthma
include particulate matter, ozone (O3), nitrogen dioxide (NO2)
and sulphur dioxide (SO2).19,20 In this regard, South African
ambient air-quality standards are not as stringent as World
Health Organisation (WHO) standards for particulate matter,
SO2 and O3.21
Airborne particulate matter consists of a mixture of small
particles of organic and inorganic compounds, acids, metal,
dust and carbon. Particulate matter is categorised according to
size; particulate matter10 includes particles smaller than 10 µm
that can bypass the nasal and pharyngeal-ltering processes
to reach the bronchioles and alveoli, while particulate matter2.5
particles are smaller than 2.5 µm and can therefore extend
further and lead to cardiopulmonary impairment.20 The primary
source of particulate matter is fossil-fuel combustion emissions,
although mine dumps are a major contributor to particulate-
matter air pollution in surrounding communities.17,22 A high
prevalence of wheeze and rhinoconjunctivitis has been found
in communities living near mine dumps.22 Particulate matter is
more common in urban environments, where there are more
conned motor-vehicle emissions and industrial exposure.20
Diesel exhaust is an important component of urban particulate
matter, with particle sizes ranging from nanoparticles to large,
coarse particles. The immediate response to exposure is similar
to a type 1 hypersensitivity response, with histamine release
within minutes of exposure and increased cellular inammation
and chemokine production occurring within hours. Long-term
exposure has been associated with possible increased allergic
sensitivity to other allergens.20 Pollen allergens may bind to
diesel-exhaust carbon particles, leading to delivery into the
lungs.5
O3 is generated at ground level by a chemical reaction of
sunlight on mixtures of NO2 and hydrocarbons from fossil-fuel
combustion.19 Major sources of O3 include vehicular trac,
power plants and industrial operations.19 O3 can increase airway
inammation and hyperresponsiveness;19 exposure to it irritates
the respiratory epithelium, is associated with decreased lung
function, increased symptoms, increased rescue-medication use
and increased risk of asthma exacerbation; and it may interact
with allergens to cause a greater asthmatic response.19,20 An
association has been found in children between O3 exposure,
wheeze and AR, as well as an increased rate of newly developed
sensitisation to outdoor allergens.23
Motor-vehicle exhaust, industry, power plants and forest
res are the main sources of atmospheric NO2.19 This gas is
associated with increased asthma symptoms, exacerbations and
hospitalisations, and also with lower-lung function.19 Long-term
exposure to NO2 exacerbates bronchitis symptoms in asthmatic
children.21 SO2 is an ambient air pollutant formed mainly by the
combustion of high-sulphur coal or oil; it can cause reduced lung
function.19
ENVIRONMENTAL CONTROL PRACTICES
Although allergen avoidance is seldom eective on its own
as a way of controlling atopic disease, it forms an important
part of its management, because reducing exposure can help
medical therapy to work more eectively. Unfortunately, pollens
and outdoor fungal spores are impossible to avoid completely.
Allergen exposure may occur in the outdoor environment
or when allergens enter the indoor environment. Allergen
avoidance is therefore based on advising patients about when
to enter the outdoor environment and about preventing outdoor
allergens from entering the indoor environment (see Table I).1
TABLE I: ENVIRONMENTAL CONTROL MEASURES
• Avoid outdoor activities during peak pollen and mould periods.
• Keep doors and windows closed and use air conditioning.
• Change clothing when returning home.
• Wash outdoor pets before they come indoors and keep them out of
bedrooms.
• Mould-sensitive patients should avoid contact with decomposing leaves,
grasses and grains and avoid camping or walking in forests.
POLLENS
Pollen levels are highest in the morning, especially on hot, dry,
windy days.1 During the pollen season, outdoor activities should
therefore preferably be planned for the late afternoon or evening.
Measures that can be taken to prevent outdoor allergens
entering indoors include keeping windows and doors closed.
Patients should remove their clothes and shower immediately
when they return home and change into clothes that are not
worn outdoors, as pollen may stick to the clothing, skin and hair.1
High-eciency particulate air (HEPA) lters can reduce pollen in
the home.9 Outdoor pets should also be kept out of bedrooms
and should be washed after spending extended time outdoors,
because they can bring allergenic pollen and mould into the
house.1 Although these measures may reduce the amount of
pollen entering the home, pollen allergens can accumulate
in house dust and remain beyond the grass pollen season.5
Frequent vacuuming is necessary to reduce this potential
source of exposure.
MOULDS
Mould spores are also most prevalent on dry, windy days, but
levels are higher in the evenings and after rain.1,3,24 Mould-
sensitive patients should avoid contact with decomposing
leaves, grasses and grains and avoid camping or walking in
forests or densely vegetated, deep-shade areas, especially
during autumn and winter.24 It is not possible to eliminate mould
from the outdoor environment, but dense vegetation close to
the house should be thinned out and dead or decaying trees
and bushes should be removed. Stagnant water near the house
should also be drained.1

14 Current Allergy & Clinical Immunology | March 2019 | Vol 32, No 1
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POLLUTION
It is hard to avoid outdoor air pollution because there is no
realistic way of avoiding exposure.8 When the air quality is poor,
patients should avoid exertion and stay indoors, preferably in an
air-conditioned environment.19 Clean-air policies are the most
eective way to reduce the health eects related to air pollution.8
CONCLUSION
Pollens and moulds are the predominant outdoor allergens
causing allergic conjunctivitis, rhinitis and asthma. Although it is
impossible to avoid these allergens completely, some measures
can be introduced to reduce exposure in order to improve
disease control.
DECLARATION OF CONFLICT OF INTEREST
The author declares no conict of interest.
This article has been peer reviewed.
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