Particulate matter isn’t just one pollutant. It’s a host of fine particles like road dust, suspended tire debris, diesel exhaust, fireplace smoke and industrial discharges in the air. You can see the bigger particles like smoke and soot, but the most harmful particles are the really small ones that you can’t see.

Particles bigger than 10 microns get stuck in your nose and throat but don’t make it into your lungs because they’re too big. Particles between 2.5 and 10 microns make it into the lungs – but mainly the upper bronchi.  Particles smaller than 2.5 microns make it deep into the lungs and are therefore more harmful.  Basically, the smaller the particle, the more harmful it is.

Many years ago, Congress passed the Clean Air Act, which gave EPA the responsibility to set air quality standards for a host of pollutants including particulate matter. State and local jurisdictions are responsible for complying with those standards.

That has been the driving force for various air quality policy interventions like vehicle testing and inspections, point source regulations (industry mostly), dust control requirements for construction sites, and even agricultural practice requirements.

Over the years, EPA has updated their standards using the best science available. Over that time, it has become more and more clear that focusing on regulating the finest particles (<PM2.5) provides the biggest public health benefit.

With that in mind, EPA has been revising their particulate matter standards over the last couple of years…  and yesterday announced new standards – strengthening the annual average air for PM2.5 from 12 micrograms per cubic meter (annual average) to 9 micrograms per cubic meter (annual average).

Once jurisdictions come into compliance, EPA estimates the change will prevent 4,500 premature deaths and 290,000 lost workdays (yielding up to $46 billion in net health benefits).

Along with strengthening the primary annual PM2.5 standard, EPA changed monitoring network design criteria to include a factor that accounts for proximity of populations at increased risk of PM2.5, advancing environmental justice by ensuring localized data collection in overburdened areas.

EPA has 2 standards for PM2.5 – the annual average standard, which EPA has revised from 12ug/m3 to 9ug/m3, and a 24-hour average standard, which EPA didn’t change. EPA also didn’t change the current primary 24-hour standard for PM10, which provides protection against coarse particles.

As a result of yesterday’s change – Maricopa and Santa Cruz Counties have moved from being compliant with EPA’s PM 2.5 standard to being out of compliance. Pinal County was already out of compliance with the former, more permissive standard.

National Annual PM2.5 Averages & Attainment Status (EPA)


County Avg Annual PM 2.5 value

 In Attainment w/new Standard (9ug/m3)?

La Paz County

4.1 Yes

Maricopa County



Pima County

6.1 Yes
Pinal County 12.4


Santa Cruz County 10.2


Yuma County 8.9


* Counties moving from current ‘attainment’ status to ‘non-attainment’ under new annual average PM2.5 standard (9ug/m3)

A jurisdiction’s compliance status is important because those that don’t meet the standards need to come up with and implement plans and submit them to the EPA with the ways they will come into compliance.  For example, Maricopa County Air Quality Department will likely need to update their rules and regulations in order to meet the new standards for PM2.5. You can see their rules and regulations here – but beware- they are more than 800 pages long!