June 21 officially marked the first day of summer, bringing in long hot days of fun in the sun. Summer is also the time when air quality alerts start popping up, warning us to avoid breathing in bad air and limit activities outside. These alerts can put a damper on our outdoor plans, but why is poor air quality unhealthy for lung physiology?

The inside surface of our lungs is covered by a fluid called lung-lining fluid that helps keep the lungs clean. The fluid traps pollutants and particles, which are swept out by fine hairs that also line the lungs. Cells from the immune system drift around in the fluid, too. These cells can neutralize pollutants by ingesting and destroying them.

Air quality alerts are triggered by high levels of particulates and a chemical called ozone. When the temperature heats up, exhaust from cars, power plants and other sources turns into ozone. This is the same molecule making up the ozone layer that shields Earth from the sun’s radiation. Ozone high above protects our health, but on the ground, it can worsen it. Ozone dissolves in the lung-lining fluid, reacting with molecules in the fluid and on the surface of the lungs’ cells. The lung cells react by releasing molecules that attract and activate immune cells, leading to inflammation.

The American Lung Association describes ozone’s effect like a sunburn in the lungs. Symptoms of breathing in too much ozone include coughing, shortness of breath and difficulty breathing. The effects of ozone may be more severe among children and the elderly and in people with respiratory ailments such as asthma. The good news is that these effects are reversible and if you stop breathing ozone, symptoms can improve in a few hours. So, mind the poor air quality warning when you see it. Avoiding bad air is good for you and your lungs!

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.