Outdoor Allergens, Pollution, and Weather

Reviewed by: HU Medical Review Board | Last reviewed: October 2021

Identifying what triggers your symptoms is an important part of asthma management. Certain components of outdoor air can trigger symptoms of asthma. Common outdoor triggers include pollen, mold, pollution, and extreme weather.

Talk to your doctor for help identifying what triggers your symptoms. They can then suggest ways to reduce your exposure to your triggers. This can help improve health outcomes and reduce the need for asthma drugs.

What outdoor allergens can trigger asthma symptoms?

Pollen and mold are the most common outdoor allergens. Many people with pollen and mold allergies have hay fever (allergic rhinitis). For people with asthma, inhaling these allergens can cause an immune reaction that narrows the airways. Symptoms are often seasonal, peaking when more pollen or mold is in the air.1,2

Trees, grass, and weeds produce tiny pollen grains to fertilize other plants. The light, dry pollen can easily travel in the wind. Different plants produce pollen during different seasons. For example, trees produce the most pollen during the spring. Grasses produce more pollen in the early summer. And weeds produce more pollen in the late summer and fall.1,3

Mold is actually a fungus, not a plant. Mold seeds (called spores) travel through the air and can cause allergic reactions. Symptoms usually peak from July to early fall. But mold can grow in many places and climates, so allergic reactions can occur year-round.1,4

Why does pollution trigger asthma symptoms?

Air pollution includes any substances in air that are not a natural component of air. Inhaling air pollutants can increase airway inflammation and decrease lung function. For people with asthma, this makes the airways more sensitive, increases asthma symptoms, and leads to worse outcomes. Long-term exposure to air pollution increases the risk of asthma in children.5,6

Most air pollution comes from traffic and power generation. The most common air pollutants include:7

  • Particulate matter (found in haze, smoke, and dust)
  • Ozone
  • Nitrogen dioxide
  • Sulfur dioxide

Ozone and other air pollutants are most common in cities, where there are more cars. Ozone levels are also higher in the summer, when there is more sunlight and low winds. These levels are also the highest from late morning to early evening.6,7

How does weather affect asthma?

Certain weather conditions can trigger symptoms of asthma, such as:1,8

  • High heat
  • Extreme cold
  • High humidity
  • Sudden weather changes
  • Rain and thunderstorms

Cold and dry air can cause the airways to narrow because they lose water. Exercise during cold weather is a common trigger because you usually inhale through your mouth during exercise. This causes the air to be dryer and cooler than when you breathe through your nose.8

On the other hand, hot and humid air can also cause asthma symptoms. Air pollutants and common allergens increase in humid air. Rain and wind from thunderstorms can break up pollen grains and keep them airborne longer. This makes pollen smaller and easier to inhale.8

Climate change also affects asthma symptoms. Increasing temperatures and more severe storms increase the risk of weather triggering asthma flares.8

How can I reduce exposure to outdoor asthma triggers?

A good way to reduce exposure is to monitor levels of your outdoor triggers. Some things to check before going outside include:

On days with dangerous levels of outdoor triggers, take steps to limit your exposure by:1,3,4,6,8

  • Limiting outdoor time when pollen or mold counts are high, AQI is above 100, or extreme weather is forecasted
  • Keeping windows closed and using an air conditioner
  • Showering and changing clothes when you come inside
  • Doing outdoor activities right after sunrise
  • Wearing a face mask, glasses, and a hat when outside, especially when doing outdoor chores
  • Using a HEPA filter indoors
  • Avoiding prolonged physical activity
  • Breathing through your nose instead of your mouth
  • Drying clothes in a dryer, not on an outdoor line
  • Talking to your doctor about over-the-counter and prescription drugs for allergies

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