As smoke from Canadian wildfires blanketed parts of the East Coast in an eerie, orange haze this week, residents in places from Vermont to South Carolina, and from Detroit to New York City, were inundated with numbers to quantify the pollution: Air Quality Index measurements that reached into the “hazardous” level of more than 300, and PM2.5 concentrations that, in New York City, topped 800. But what do those numbers mean?

Air Quality Index, or AQI, and PM2.5, particulate matter which measures 2.5 micrometers or less, are not the same thing. But both do refer to how polluted the air is, just in different ways.

What does hazardous air quality mean?

AQI, an index from the Environmental Protection Agency, is more of a “yardstick” measurement, according to AirNow, a government site that reports air quality information. AQI ranges from 0 to 500, with six different bands of AQI value equating to different levels of health concerns. A level of 0 to 50 is good, with little or no risk from air pollution; 51 to 100 is moderate, with an acceptable air quality; and so on. New York City’s AQI topped 400 in some areas on Wednesday, marking a record for the worst air pollution recorded in the city’s history. A measurement of 300 and higher is considered “hazardous,” with everyone likely to be affected by the health impacts of that bad air.

That polluted air can be made up of a few things, because AQI looks at five major air pollutants: ground-level ozone, particle pollution (also called particulate matter, and which includes both PM2.5 and PM10, particles 10 microns or less), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA has national standards for each of those pollutants, per the Clean Air Act. The EPA measures AQI with monitors across the country, and AirNow publicly reports that data. But there are other organizations that also measure AQI, including companies like PurpleAir and IQAir, which make sensors for personal use that collect air quality measurements at the hyper-local level, so community members can get granular data.

So PM2.5 is one type of pollutant, and it was singled out as cities dealt with the wildfire smoke this week—for good reason. Wildfire smoke contains lots of these particles, and “fine particles from wildfire smoke are of greatest health concern,” per the EPA. Particles 10 micrometers or smaller can enter our nose and lungs, and finer particles, of 2.5 micrometers or smaller, can even enter our bloodstream.

From there, they can make asthma more severe and worsen chronic lung conditions such as COPD and emphysema; there’s also evidence these particles can increase heart attacks, strokes, and lead to premature deaths. In children, PM2.5 exposure has been linked to impaired cognitive development. In the short term, PM2.5 can have immediate effects as well, like irritating your eyes, nose, throat, and lungs, and causing coughing, a runny nose, sneezing, and shortness of breath.

When New York City reported that PM2.5 hit above 800 in some areas on Wednesday, it was an astounding level that one healthy air expert, Harvard professor Joseph Allen, said he never thought he would see on the East Coast. That number was a measurement of the micrograms of these particles per cubic meter of air. The National Ambient Air Quality Standard says that for a 24-hour period, a level of 35 micrograms per cubic meter of air is safe. Over a longer period, like an annual average, that standard is 12 micrograms per cubic meter of air.

PM2.5 don’t only come from wildfire smoke. These particles are a dangerous pollutant also prevalent in vehicle exhaust, and from burning fuels like wood, oil, or coal, which means they also can come from power plants; these particles have also been found in cooking emissions. When fine particulate pollution is extremely high, experts recommend people reduce their time spent outside, avoid intense activity, use air filters, or wear respirator masks.

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