Ozone (O₃) might sound like a good thing—it protects us in the upper atmosphere—but at ground level, it’s a harmful pollutant. Outside, it forms when sunlight reacts with emissions from cars and industry. But it can easily make its way indoors, especially during hot weather or on “ozone alert” days.

Ozone also comes from indoor sources, including:

• Certain air purifiers that generate ozone on purpose (often marketed as “ionizers” or “ozone generators”)
• Copy machines, laser printers, and electronics
• UV air cleaners or older corona-discharge equipment