The summer heat is often assumed to trigger asthma attacks. But the heat itself is usually not the cause of attacks, said Richard Nicklas, clinical professor of medicine at the George Washington University Medical Center. In fact, patients with exercise-induced asthma often "have more trouble when [the] weather is dry and cool than when it's warm and humid," he said.

"There isn't a whole lot [of evidence] in regard to the heat directly affecting people with asthma," Nicklas said. But because many people with asthma have allergic triggers -- some of which are present in higher amounts during warmer weather -- they sometimes experience attacks during the summer months. Among those triggers:

* Allergens. Dust mites, a common household allergen, tend to breed heavily during hot, humid weather, Nicklas said. Mold -- fungi that reproduce by sending spores into the air -- also increases during damp, warm weather. Pollen counts vary year round, depending on the type of grass, tree or weed, according to the National Institute of Allergy and Infectious Diseases, but some plants pollinate heavily during the summer months.

* Air Pollution. High levels of air pollution -- including ozone (present in smog) and particle pollution (found in smoke, haze and dust) -- can cause asthma symptoms, such as coughing, wheezing and difficulty breathing, according to the Environmental Protection Agency (EPA). Symptoms may appear up to a day after exposure to polluted air. Pollution can also sensitize people with asthma to other triggers, such as mold and dust mites, according to the EPA.

Ozone is often found in high amounts on hot summer afternoons and evenings. People with asthma should try to stay indoors during these peak times when "orange" and "red" ozone alerts are issued.

Particle pollution may be bad at any time of year, reports the EPA, particularly near factories and "when there is smoke in the air from wood stoves, fireplaces or burning vegetation," close to busy roads during rush hour, and during calm weather, when pollution can build up.

Track your triggers. The National Allergy Bureau, part of the American Academy of Allergy, Asthma and Immunology, counts airborne pollens and mold spores at 75 stations in the United States, including the Walter Reed Army Medical Center in the District.

The EPA lists air quality forecasts on the Web, at, that describe "how clean or polluted your air is, and what associated health effects might be a concern for you," reports the Web site.

-- January W. Payne