Outdoor Allergens, Pollution, and Weather

People with well-controlled asthma rarely need to avoid the outdoors.1 However, outdoor air can contain pollen, mold, and pollution, which may cause a reaction in some people with asthma. Additionally, some types of weather can be a trigger for asthma, particularly when exercising.2

How do pollen and mold affect asthma?

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, such as ragweed, pollinate in late summer until the first frost.3

Seasonal mold (fungal) spores can also trigger asthma symptoms.2 Outdoor mold lives on rotting trees and leaves. Outdoor mold usually peaks in June and decreases after the first frost.3 Several studies have found that 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, 7.7 percent of all adults have hay fever.4 Slightly more women than men are affected (8.3 percent vs. 6.4 percent). People ages 45 to 64 are the most affected.4 Hay fever occurs most frequently in the South and Northeast.4

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

You can check the National Allergy Bureau pollen and mold counts) each day. On days with high pollen and mold counts, you can take steps to limit your exposure:1-3,5-6

  • 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

Asthma and air pollution

Breathing in pollutants increases airway inflammation and decreases lung function.2,6 The most problematic pollutants are ozone and airborne particles, which are found in haze, smoke, and dust.7. These pollutants have been shown to increase asthma symptoms, use of rescue inhalers, emergency department visits, and hospitalizations.2 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 Ozone levels are highest in the summer, from late morning until early evening.6 One study found that exposure to air pollution in children between birth and age 2 has been associated with coughing and wheezing.8 Some studies have also found that long-term exposure to air pollution increases the risk of >asthma in children.9,10

How many people are affected by air pollution?

In the United States, about 82 million people live in counties where pollution levels were higher than national air quality standards in 2019.11 The effects of pollution vary from person to person, even in healthy people. The same level of air 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

How do I reduce my asthma symptoms on days with high air pollution?

You can find information about daily pollution levels in your area from 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 also affect indoor air quality
  • 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 include how much air pollution affects your asthma symptoms, the importance of being active outside, and indoor options for being active.

How does weather affect asthma?

Extreme weather conditions often irritate asthma symptoms more than mild weather. Climate change also increases asthma symptoms.12

How does cold air affect asthma?

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?

One study found that cold air causes breathing problems in 69 percent of men and 78 percent of women with asthma.13 In comparison, only 18 percent of men and 21 percent 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.14

How do heat and humidity affect asthma?

Moist air feels heavier and denser to breathe. Breathing hot, humid air can cause your airways to narrow and make you feel short of breath.15 Hot, humid weather also causes many people to sweat. This can make you dehydrated and cause you to breathe faster. This can trigger asthma symptoms.15

Hot, humid weather also affects air quality. Moisture in the air traps pollutants that are known to cause asthma symptoms.7 Common allergens like dust mites and mold thrive in humidity, which may lead to asthma symptoms.2

How common is it to react to heat and humidity?

During the hot summer months, a humidity level of less than 55 percent is “comfortable.” Humidity between 55 to 65 percent begins to feel “sticky,” and anything above 65 percent is considered “oppressive.”16

One study found that after spending 4 minutes in hot, humid air (120.°F and 75 to 80 percent humidity), people with asthma had a 112 percent increase in airway resistance. In comparison, only 22 percent of people with healthy lungs experienced airway resistance.17

Another study found that a 10 percent rise in humidity led to an increase in emergency room visits for asthma symptoms. A 10-degree rise in outdoor temperature also increased hospital visits 1.8-fold.18

What can I do to reduce my symptoms in hot and humid weather?

The main way to reduce asthma symptoms is to stay indoors during the mid-day. Heat, humidity, and air pollution levels are often worse at that time of the day.19

If you can, use an air conditioner to cool down your home. Using a fan and keeping drapes and windows can also keep your home cool. A dehumidifier can also control humidity levels inside your home.19

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Written by: Sarah O’Brien & Heather Morse | Last Reviewed: July 2020