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Outdoor Air (Weather, Pollution)

People with well-controlled asthma rarely need to avoid the outdoors.1 However, outside air can contain pollen, mold, and pollutants. These allergens and irritants may cause a reaction in some people with asthma. Additionally, cold weather can be a trigger for asthma, particularly when exercising.

Pollen and mold

Pollen and mold are common outdoor allergens. When an allergen enters the airways of a person with asthma, it sets off a reaction that causes the airways to narrow. People with pollen or mold allergies usually also have symptoms of hay fever (allergic rhinitis): sneezing; itchy eyes; runny, itchy, and stuffy nose.2 If you are allergic to pollen and mold, your symptoms will get worse during peak pollination times.

Pollen is a fine powder produced by trees, grass, and weeds.3 Each plant produces pollen at a different time of the year. Trees pollinate in the spring (March through June). Flowering trees, such as birch, oak, elm, and maple, produce the most pollen. Grasses pollinate in the early summer (May through June). Weeds pollinate in late summer until the first frost.

Seasonal mold (fungal) spores can also trigger asthma symptoms.2 Outdoor mold lives on rotting trees and leaves. Outdoor mold peaks in June and decreases after the first frost.3 One type of mold called Alternaria is associated with severe asthma.3 Other types of mold can also cause asthma symptoms.

How common are pollen and mold allergies?

In the United States, 7.3% of all adults have hay fever.4 Slightly more women than men are affected (7.9% vs. 6.6%). People age 45 to 64 are the most affected. Hay fever occurs most frequently in the South and West.

What can I do to reduce my exposure to pollen and mold?

The National Allergy Bureau reports pollen and mold counts ( On days with high pollen and mold counts, you can take steps to limit your exposure:1,2,3,5

  • Stay inside during the midday and afternoon, which are peak pollen times.
  • Try to do outdoor activities shortly after sunrise, when pollen levels are lowest.
  • Wear a mask while cutting the grass or raking leaves.
  • Keep doors and windows shut, and use an air conditioner.
  • Shower and change your clothes when you come inside to remove pollen from your hair and body.
  • Use a HEPA filter indoors to reduce circulating pollen and mold spores.
  • Replace air conditioner filters regularly.
  • Use a clothes dryer instead of hanging clothes to dry outside, where they can collect pollen.

Air pollution

Breathing in pollutants increases airway inflammation and decreases lung function.2,6 The most problematic pollutants are ozone and particle pollution. These pollutants have been shown to increase asthma symptoms, use of rescue inhalers, emergency department visits, and hospitalizations.2 Levels of these pollutants are also most likely to go above national air quality standards.7 Sulfur dioxide and nitric oxide are two other pollutants that make asthma worse.2

Urban areas produce the most ozone, although ozone can travel hundreds of miles.6 One recent study showed that children living close to busy streets had more asthma attacks and chest colds than children living in greener areas.8 Ozone levels are highest in the summer, from late morning until early evening.6

It is not clear whether air pollution can cause asthma to develop. Exposure to air pollution between birth and age 2 years has been associated with cough and wheeze.9 However, a large study in Europe showed that children who lived in polluted areas were no more likely than other children to have asthma.10

How many people are affected by air pollution?

In the United States, approximately 124 million people live in counties where pollution levels were higher than national air quality standards in 2010.7 The effects of pollution vary from person to person, even in healthy people.6 The same level of pollution can trigger an asthma attack in some people, but not others. Older adults and people with heart disease are at the highest risk.6

What can I do to reduce my symptoms on days with high air pollution?

You can find information about daily pollution levels in your area at Airnow ( Air quality is measured with the Air Quality Index (AQI). An AQI above 100 is considered “unhealthy for sensitive groups,” such as people with asthma.6 On these days, you can take steps to limit your exposure to pollution:6

  • Limit outdoor time.
  • Be active outdoors in the morning when pollution levels are lower.
  • Limit or avoid prolonged physical activity, depending on how high pollution levels are.
  • Limit indoor activity on days with very high particle pollution, because particle pollution can affect indoor air quality as well.
  • Breathe through your nose. Breathing through your nose helps to filter the air, which limits the number of pollutants that enter your lungs.

It is important to weigh the benefits of staying indoors against the risks of reduced physical activity.6 Considerations are: how much air pollution affects your asthma symptoms, the importance of being active outside, and indoor options for being active.

Cold air

When the outside air is cold and dry, the lungs lose heat and water with each breath.2 These losses can trigger airway narrowing, especially during exercise. Having a mild cold while doing strenuous cold-weather exercise can cause asthma symptoms.

How common is it to react to cold air?

Cold air causes breathing problems in 69% of men and 78% of women with asthma.11 In comparison, only 18% of men and 21% of women with healthy lungs have trouble breathing in the cold. Among people with asthma, symptoms start when the temperature drops to about 36°F.11

What can I do to reduce my symptoms in cold weather?

The primary way to reduce cold-related symptoms is to warm the air before you breathe it in. You can cover your nose and mouth with a scarf or face mask.2

Written by: Sarah O'Brien | Last Reviewed: May 2016.
  1. Global Initiative for Asthma. Global Strategy for Asthma Management and Prevention 2014. Accessed 11/12/14 at:
  2. National Heart, Lung, and Blood Institute. Expert panel report 3 (EPR-3): Guidelines for the diagnosis and management of asthma - Full Report 2007. Accessed 11/12/14 at:
  3. Baxi SN, Phipatanakul W. The role of allergen exposure and avoidance in asthma. Adolesc Med State Art Rev. 2010;21:57-71.
  4. Centers for Disease Control and Prevention. Summary Health Statistics for US Adults: National Health Interview Survey, 2012. Accessed 1/10/15 at:
  5. American College of Allergy, Asthma, and Immunology. Pollen Allergy – Pollen and Mold Counts. Accessed 1/10/15 at
  6. Laumbach RJ. Outdoor air pollutants and patient health. Am Fam Physician. 2010;81:175-180.
  7. Environmental Protection Agency. Highlights: Our Nation's Air - Status and Trends through 2010. Accessed 1/10/15 at:
  8. Esposito S, Galeone C, Lelii M, et al. Impact of air pollution on respiratory diseases in children with recurrent wheezing or asthma. BMC Pulm Med. 2014;14:130.
  9. Wright LS, Phipatanakul W. Environmental remediation in the treatment of allergy and asthma: latest updates. Curr Allergy Asthma Rep. 2014;14:419.
  10. Mölter A, Simpson A, Berdel D, et al. A multicentre study of air pollution exposure and childhood asthma prevalence: the ESCAPE project. Eur Respir J. 2014 Oct 16 [ePub ahead of print].
  11. Harju T, Mäkinen T, Näyhä S, Laatikainen T, Jousilahti P, Hassi J. Cold-related respiratory symptoms in the general population. Clin Respir J. 2010;4:176-185.