Ground-level ozone forms when traffic and industrial emissions react in sunlight. It’s the main component of summertime smog and can irritate lungs and trigger asthma. Here’s how it forms, when it’s worst, and what you can do.

Nitrogen dioxide is a combustion gas from vehicles, gas stoves, and power plants that can irritate the lungs and worsen asthma. Here’s where it comes from, why indoor sources matter, and simple ways to reduce exposure.

Acid rain is precipitation made more acidic by air pollution, mainly sulfur dioxide and nitrogen oxides from burning fossil fuels. It has damaged lakes, forests, and buildings, but emissions cuts have greatly reduced the problem in many regions. Here’s how it forms and what’s changed.

PM2.5 is fine particulate matter small enough to reach deep into the lungs and bloodstream. It’s linked to heart and lung problems, and wildfire smoke is a major and growing source. Here’s what it is, where it comes from, and how to reduce exposure.