Vox – no paywall: “It’s not enough to trust the senses to know when it’s a bad air day. Well before you can see or smell smoke, it can start wreaking havoc on the lungs. That haze you can see and smell on a particularly polluted day is made of ozone and fine particulate matter. Fine particulate matter, known as PM2.5 (the 2.5 microns describes its size, 30 times smaller than the width of a human hair) can embed in the cells of the lung and the bloodstream, aggravating inflammation, asthma, heart disease, and mental health. And ozone causes similar damage. In the stratosphere, ozone blocks ultraviolet radiation from the sun, but at ground level it can cause shortness of breath and damage to respiratory tissue. Both pollutants can affect the entire body in all stages of life: young and old, and even the developing fetus. They come from sources as varied as the tailpipe of a truck, your neighbor’s barbecue, coal plant, or an incinerator. PM2.5 is capable of traveling thousands of miles across the world on the wind — taking, for instance, about five days to reach the US from China. The dose makes the poison; there is a difference between moderately bad air and really bad air. Public health experts recommend monitoring changes in air quality as often as you check the weather. But you should also know some basic facts to help you determine your own sensitivity to air pollution and the appropriate action to take…

You can find the latest AQI on the EPA’s AirNow website or by downloading its AirNow app. Weather apps are often using propriety data from a company called BreezoMeter to determine AQI and forecasts. These numbers are based on EPA monitoring but may not be identical to the EPA’s AQI, though they should be in the same ballpark. Outside the US, air pollution monitoring can vary widely depending on the country, so AQIs reported around the world may also be pulling from a mix of computer modeling and satellite data…”