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ACHD, Stop Messaging Like It’s 1999: Why EPA’s NowCast AQI is Essential to Public Health

Updated: Jul 26, 2023

As wildfire smoke enveloped our region at the end of June and air quality plummeted, we heard (and thought) one question again and again: Is it safe to go outside?

When conditions were at their worst, the simple answer was, “No, not really,” and for a time the EPA’s AirNow platform and Allegheny County Health Department (ACHD) outreach agreed that levels of fine particulate matter (PM2.5) were “very unhealthy,” meaning that the “risk of health effects [was] increased for everyone.”

To our dismay, the EPA and ACHD appeared not to agree on the messaging or the Air Quality Index (AQI) a great deal of the time over those three hazy days. Quite often, AirNow values and those ACHD were sharing didn’t line up.

The two agencies’ dueling approaches stem from differences in how hourly data is averaged, as well as the very goal of reporting the AQI to the public. From our perspective, the EPA and ACHD taking two different approaches to public outreach when air quality was “very unhealthy” was very unacceptable and potentially dangerous.

Actual people with health conditions or vulnerable loved ones were scared, and the two agencies with expertise in both air quality and public health couldn’t get on the same page. What should have been a clear, unified message to the public about the risks of exposure to harmful levels of PM2.5 occasionally descended into color-coded confusion.

Public reprimands aside, we want to provide a deeper explanation of why both agencies technically were reporting valid numbers, and also why the EPA was reporting FAR more useful information under the circumstances.

Some Necessary Background Info

The health-based National Ambient Air Quality Standard for PM2.5 has two components: a daily (24-hour) standard and an annual standard. Even if air quality exceeds those numeric limits – 35 micrograms per cubic meter (ug/m3) and 12.0 ug/m3, respectively – that does not mean air quality violates the standard.

The actual regulations state that an area fails to meet the annual standard if the annual concentration averaged over three years exceeds 12.0 ug/m3. Similarly, violating the daily standard requires roughly eight days per year, averaged over three years, to be over 35 ug/m3.

If you’re thinking, “That’s a lot of numbers; just tell me what’s healthy and what isn’t,” you aren’t alone.

In 1999, the EPA created the Air Quality Index (AQI) to address the complexity of the standard and replace a collection of varying regional approaches. The AQI provided a simple, color-coded way to report past conditions – “there were ‘X’ number of ‘red’ air quality days last month” –and forecast future conditions – “we expect ‘orange’ air quality tomorrow; open burning will be prohibited.”

This was a very important step toward improving public health communications because it created a single, national, uniform means of providing accurate, timely, and easily understandable information about daily levels of pollution. However, one problem inherent in the AQI as originally conceived was that it was not set up to address real-time conditions.

The Research Improved and the Internet Got Faster

This might sound trite, but a lot has changed since 1999. Feel free to reminisce a bit (or ask the internet what it was like way back then), but particularly relevant to this story is that both the internet and public health research have *really* come a long way since the waning days of the previous millennium.

For perspective: The first ever EPA standard for PM2.5 was only finalized in 1997 and at least two GASP contributors to this story could still get email via a dial-up modem in 1999.

Now that public health officials have decades of research on PM2.5 and the infrastructure exists to monitor and share PM2.5 levels reliably on an hourly basis, updating the 1999 approach to how EPA shares that information became necessary.

But there’s one small catch: The extensive, federal, complicated, PM2.5 standard is based on a yearly average and a 24-hour average. You don’t need to be a mathematician to realize a current or real-time PM2.5 level outside is not the average of the previous 24 hours.

We graphed the data so you can see for yourself:

The solution was EPA’s NowCast AQI (which are the values reported on the AirNow website and app).

EPA’s approach is based on a fundamental principle of public health: reducing exposure to potentially harmful substances reduces the risk of an adverse impact on human health.

According to an EPA AirNow Knowledgebase article, EPA’s goal was “to report shorter-term data to caution people in time for them to reduce their 24-hour exposure. The NowCast is EPA's method for relating hourly data to the AQI.”

In a passage particularly relevant to our hazy June days, the EPA stated that the “NowCast is designed to be responsive to rapidly changing air quality conditions, such as during a wildfire.” The agency continued: “The NowCast allows AirNow's current conditions to align more closely with what people are actually seeing or experiencing.”

In no uncertain terms, current information is crucial for a concerned public. Can my kids go out and play? Should I check on my grandma? The bus is running late; is it safe to wait outside? The questions people have when conditions deteriorate and/or change rapidly cannot be answered with a view to the previous 24 hours.

And this isn’t just some theoretical exercise for a healthier approach to living. Even though the official PM2.5 standard doesn’t include hourly or other shorter-than-a-day limits, the EPA has acknowledged some studies have shown adverse effects of exposure to levels of PM2.5 over 120 ug/m3 for two hours.

When air quality is exceptionally poor, knowing how bad conditions are is very important, and as the EPA put it, the NowCast, “gives people information they can use to protect their health when air quality is poor – and help them get outdoors and get exercise when air quality is good.”

Does this mean ACHD was wrong? Only in its approach to public communications.

In the same Knowledgebase article we’ve been quoting, the EPA acknowledged “it is not valid to use shorter-term (e.g. hourly) data to calculate an AQI value.” The NowCast AQI published on does not supersede or invalidate the older approach, it is simply a tool to help the public understand the current conditions.

If ACHD publishes an AQI value based on 24 hours of data, that is a valid calculation of the AQI. It just also happens to be of little use for an active, connected, and concerned public.

To ACHD’s credit, it added a notice to its website expressly stating that it reports a 24-hour average value, but it then recommended readers access raw data not presented in the AQI format for shorter-term considerations.

Call us crazy, but we think promoting is a superior option to ACHD assigning the public math homework before they go outside.

As we see it, prevention is the best medicine, and ACHD’s approach isn’t helping. Failing to promote modern and understandable tools meant to protect public health is absurd, and neither the EPA nor ACHD can state sanely that publishing different versions of the AQI serves the public’s best interest.


Did you find this blog helpful? We hope so. Truth be told: We think about you all the time. Seriously. Our staffers get up early when we know air quality is expected to be abysmal so we can review the data, crunch the numbers, and let you know what to expect when you start your day. We attend subcommittee meetings, hearings, and webinars because we pride ourselves on being your eyes and ears on all things air quality. Help us continue to show up for all our fellow breathers by making a $5 donation today.

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