High above us, stratospheric ozone forms the ozone layer, blocking the sun’s harmful ultraviolet rays. That’s the good ozone. And the good news is that the hole in the ozone layer is diminishing, thanks to a full ban of CFCs and other ozone destroying chemicals since 2010.

Tropospheric ozone, also known as ground-level ozone, is the bad news. Bad for your health, bad for animals and plants, and bad for the environment. Even though ozone levels in the US on average are below the safety level established by the EPA of 70 PPB (parts per billion), that rate is frequently exceeded in problem areas. In the US, most of the cities with the highest ozone levels are in the southwest and west, as the sunny, dry climate is favorable to ozone formation. But ozone problems can crop up anywhere. 

Worldwide, East Asia, the Persian Gulf, India, northern South America, the Gulf of Guinea, and Malaysia/Indonesia have the highest ozone levels, although local conditions in other regions can also be poor.

“Breathing in other pollutants in the air may make your lungs more responsive to ozone—and breathing ozone may increase your body’s response to other pollutants. For example, research warns that breathing sulfur dioxide and nitrogen oxide can make the lungs react more strongly than just breathing ozone alone. Breathing ozone may also increase the response to allergens in people with allergies.” American Lung Association

For more information about the dangers of ambient ozone,  go to the California Air Resources Board ozone information. page.

On Ozone Alert days, one suggestions is to stay indoors. That can make sense when the air inside is better than the air outside. However, some research suggests that even with windows closed, ozone indoors can reach 25 to 50% percent of the outdoor level. If ozone is a problem outside, it can easily become one indoors, too. Ozone can be high indoors for two reasons: ozone infiltration and new indoor sources of ozone formation. Infiltration happens easily when doors or windows are left open, allowing ozone to get inside. Even with the windows closed, infiltration can happen if the building is “leaky,” you open the door frequently, or outside air is used for ventilation. Infiltration ozone can be hard to avoid as HVAC systems do not inherently reduce it. New ozone formation indoors occurs due to a range of products including ozone generating air purifiers, some water purifications systems, photocopiers, and laser printers.

Ozone reacts with chemicals found in various products to produce other potentially toxic substances like formaldehyde, exacerbating its overall toxicity…Conversely, the presence of other air pollutants like particulate matter and sulfur dioxide can worsen the respiratory and cardiovascular impacts of ozone exposure. These pollutants likely inflame and damage the airways, making the lungs more vulnerable to ozone’s oxidative effects.

Source: National Institutes of Health