Ozone. It’s an invisible gas but it pollutes the air and may make it harder for you to breathe.

Ozone is created when sunlight triggers a chemical reaction between oxygen-containing molecules and pollution that comes from cars, power plants, factories, and other sources, says the NIH News in Health. Hot weather is one of the triggers for the creation of ozone. It’s not as much of a problem in winter, say the NIH scientists.

High levels of ozone might cause you to cough or have throat irritation. Your eyes might water, and you may have trouble breathing. The ozone irritates the lining of your airways and lungs. If you have a lung condition such as asthma, the damage can be greater.

Adjust your activities on a bad air day to reduce your exposure to air pollution and stay indoors. Limit outdoor activities. Even if you’re in good health, mow the lawn or take a run or walk later in the evening or first thing in the morning. Ozone levels tend to peak between mid-afternoon and early evening. You can track air quality at www.airnow.gov.