Why Your AQI Reads Higher Now Than It Did in 2023

If you check an air quality number before deciding whether to run outside, open the windows, or send the kids to the park, you have probably noticed more yellow days than you used to see. Your town is not necessarily dirtier than it was in 2023. The ruler changed.

On May 6, 2024, the concentrations behind the AQI categories moved. Air that reported as Good three years ago can report as Moderate today with the exact same particle count in it. Here is what changed, how to redo the conversion yourself, and what the new number is actually telling you.

What EPA actually changed

The trigger was the particle pollution standard. EPA's overview of the February 2024 final rule states that it revised the level of the primary annual PM2.5 standard from 12.0 micrograms per cubic meter to 9.0, while keeping the 24-hour standard at 35.

The AQI had to follow, because the boundary between Good and Moderate was pinned to that standard. EPA's fact sheet on the final AQI updates lays out the old and new breakpoints side by side, and notes the previous ones had been in place since 2012.

Good used to cover 0.0 to 12.0 micrograms per cubic meter. It now stops at 9.0. Moderate used to start at 12.1 and now starts at 9.1.

The top of the scale moved too. Unhealthy used to run to 150.4 and now stops at 125.4. Very Unhealthy used to run to 250.4 and now stops at 225.4. Hazardous was two stacked bands and is now a single one starting at 225.5.

One band did not move at all. Unhealthy for Sensitive Groups still runs from 35.5 to 55.4, because EPA kept the 24-hour fine particle standard at 35 micrograms. This was a change to the particle pollution breakpoints, not to ozone, carbon monoxide, nitrogen dioxide, or sulfur dioxide.

The same air, two different numbers

Say a monitor near you reads 11.0 micrograms per cubic meter over 24 hours. That is an ordinary day in a lot of places, nothing dramatic.

Under the old breakpoints that sat inside Good, which ran from 0.0 to 12.0 and mapped to AQI 0 to 50. Eleven micrograms is 92% of the way up that band, so the reported AQI was 46, and the colour on the map was green.

Under the current breakpoints the same 11.0 falls in Moderate, which runs from 9.1 to 35.4 and maps to AQI 51 to 100. It sits near the bottom of that band, so the reported AQI is 55, and the colour is yellow.

Nothing about the air is different in those two sentences. One reading is 46 and green, the other is 55 and yellow, and both describe the same eleven micrograms of particulate.

How to do the conversion yourself

This is worth knowing if you own a PM2.5 sensor, because most of them show you raw micrograms and leave the translation to you. EPA's technical assistance document for reporting the daily AQI gives the method in four steps, and it is arithmetic you can do on a phone.

Truncate your PM2.5 concentration to one decimal place. Find the two breakpoints that bracket it. Then slide linearly between the AQI values those breakpoints correspond to, and round to the nearest whole number.

The current PM2.5 bands, in micrograms per cubic meter over 24 hours, are 0.0 to 9.0 for AQI 0 to 50, then 9.1 to 35.4 for 51 to 100, then 35.5 to 55.4 for 101 to 150, then 55.5 to 125.4 for 151 to 200, then 125.5 to 225.4 for 201 to 300, and 225.5 and up for 301 and beyond. An AQI of 500 corresponds to 325.4.

Working the 11.0 example through: the band is 9.1 to 35.4 wide, which is 26.3 micrograms, and it covers 49 AQI points from 51 to 100. Your reading is 1.9 micrograms above the bottom of the band, so you are 1.9 divided by 26.3 of the way up it, which is 3.5 AQI points. Add that to 51 and you get 54.5, which rounds to 55.

The wildfire end of the scale moved further

The low end got most of the attention, but the upper breakpoints shifted by much larger amounts, and that is where a category jump changes what people are told to do.

Take a smoky afternoon at 140 micrograms per cubic meter. Under the old breakpoints that landed in Unhealthy at AQI 195, red on the map. Under the current ones it lands in Very Unhealthy at AQI 215, purple on the map.

EPA said to expect exactly this. Its fact sheet predicts many areas will see more Moderate days because of the change, no significant increase in the other categories from the breakpoints alone, but wildfire events shifting some days from Unhealthy to Very Unhealthy, or from Very Unhealthy to Hazardous.

No, your air did not get worse

EPA answers this one directly in the same fact sheet, under the question of whether the AQI was updated because air quality is getting worse. The answer is no. The breakpoints moved because the agency learned more about the health effects of breathing particle pollution, and because the annual standard itself was tightened.

The practical consequence is that any mental baseline you built before mid-2024 is now miscalibrated. If you remember your area sitting around 40 most days and it now sits around 55, you are looking at a rescaled version of the same air, not a decline.

The comparison that still works is micrograms against micrograms. If you want to know whether your air has genuinely changed since 2022, compare concentrations across years rather than AQI values, because the concentration scale did not move.

Why your phone app and the official number disagree

There is a second reason the number in front of you may not match what you expect, and it has nothing to do with the 2024 change.

The official daily AQI takes a full 24 hours of data, which means in practice it describes yesterday. The live number on AirNow and most apps is the NowCast, a different calculation designed to respond faster when conditions change quickly.

The technical assistance document describes the PM NowCast as a weighted average of the previous 12 hours. When air quality is stable the hours are weighted fairly evenly, and when it is changing fast the most recent hours dominate, approaching a three-hour average. It needs two of the last three hours of data to produce a value at all.

So the live reading is built to answer "should I go out in the next hour" and the daily figure is the record for the day. During a fast-moving smoke event they can differ by a lot, and neither is wrong.

One more reporting change came in with the same rule: metro areas above 350,000 people now have to report the daily AQI seven days a week, up from the five-day requirement set in 1999.

What to do with the number once you have it

The category boundaries themselves did not change. Good is still 0 to 50, Moderate 51 to 100, Unhealthy for Sensitive Groups 101 to 150, Unhealthy 151 to 200, Very Unhealthy 201 to 300, Hazardous above that. Only the concentrations behind them moved.

That means the guidance attached to each band is unchanged as well. Our AQI Health Advice Calculator takes a reading and a sensitivity profile and returns the EPA activity guidance for that band, and its Moderate entry still tells the general public that normal outdoor activity is fine, while suggesting sensitive groups ease off prolonged or heavy exertion if they notice symptoms.

At the Very Unhealthy band our tool reads differently: general public should avoid prolonged or heavy exertion, and sensitive groups should avoid outdoor physical activity and stay indoors with the windows closed. That is the band a smoky 140 micrograms now lands in rather than one step lower.

Worth knowing which group you are in. For particle pollution the technical assistance document lists the sensitive groups as people with heart or lung disease, older adults, and children including teenagers, and it notes EPA recommends adding pregnant people during a smoke event.

Their guidance kicks in at lower readings than the general-public advice does. What any of it means for your own condition is a conversation for your healthcare provider, not for a colour on a map.

If you kept a personal rule keyed to the colour rather than the number, that rule quietly got stricter in May 2024 without you agreeing to it. Worth deciding on purpose whether you want it that way.