Air quality information

The air quality index (AQI) is a unitless number that presents the concentration of a specific pollutant relative to an EPA-established outdoor air quality standard. AQI ranges are established to communicate potential health effects and recommendations. In the case of wood smoke, the pollutant of concern is PM2.5 which is particulate matter (PM) less than or equal to 2.5 micrometers in diameter.

For all pollutants, the AQI of 100 is equal to the national air quality standard, which represents outdoor air quality that is acceptable for all individuals. Note that both the air quality standard and the AQI are based on a 24-hour average outdoor concentration. The AQI is also non-linear, meaning that the AQI changes in a non-linear way relative to concentration of pollutant. For example, if the PM2.5 concentration doubles, the AQI increases but by a factor of less than two depending on where the number lies on the AQI scale.

Air quality index (AQI) information from AirNow.gov

Using AirNow during wildfires

AirNow uses the official EPA air monitors, sometimes called federal reference method (FRM) monitors. These are mass-based monitors that use various technologies to measure the mass of the particulate pollution (PM2.5) and compares that to the air quality standard. These are averaged over 24 hours to compare with the standard and give an idea of how much pollution we are exposed to in outdoor air. There are very few of these monitors in the Reno-Sparks area and the best resolution monitors provide one-hour time resolutions, with some others being lower.

PurpleAir uses small, inexpensive optical sensors that count the number of particles in a particular size range and then adjust for density to calculate a mass concentration (PM2.5). This is a fundamentally different technology than the FRM. Left to their factory calibration, these devices consistently overestimate the PM2.5 for wood smoke. To adjust for this, the EPA has developed a correction factor (called “US EPA” on the PurpleAir site) that must be applied when woodsmoke dominates the pollutant load in the area, such as for smoke generated by wildfires. With this correction applied, PurpleAir PM2.5 concentrations and AQI values are comparable.

AirNow fire and smoke map (correction factor applied)

The two primary sources for air quality information are AirNow and PurpleAir. AirNow is the EPA site that uses the relatively few PM2.5 monitoring sites in the Reno-Sparks area and provides wide area summaries based on 24-hour data averages. It also provides forecasts on a daily basis. One note of caution is that AirNow reports AQI for three pollutants – ozone, PM2.5 and PM10. The highlighted AQI only represents the pollutant that results in the highest AQI at that moment. During times when wildfire smoke dominates our air, the highest AQI value is likely to be PM2.5, but it can be ozone. AirNow does specify which pollutant is highest and also reports the other two pollutants. 

By definition, the AQI is calculated using data from a 24-hour period. That's because the science we have about air pollution exposure and health is based on 24 hours, and therefore, EPA's air quality standards are based on 24 hours. It is not valid to use shorter-term (e.g., hourly) data to calculate an AQI value. However, shorter-term data is useful to caution people so that they may reduce their 24-hour exposure. The NowCast is EPA's method for relating hourly data to the AQI.

Current air quality in Reno/Sparks from AirNow

PurpleAir only reports information on PM2.5. When woodsmoke from wildfires dominates the area, it is imperative that the US EPA correction factor be applied to the measured values. This setting can be selected on the PurpleAir site.

PurpleAir sensors near the University (US EPA correction factor applied, 1 hour sampling)

This varies across campus depending on the building. Most newer buildings have excellent filtration and do provide a much lower concentration of smoke particles inside as compared to outside. Buildings like the Joe Crowley Student Union, the Mathewson-IGT Knowledge Center, and other newer buildings would be good places to meet or study when the outdoor AQI is elevated.

Some buildings are not as efficient, but all of them do provide some level of filtration compared to outside. Facilities Services does try to control the air handling systems (where they can) to reduce incoming outdoor air (and therefore smoke), but unfortunately, this means more recirculation of air inside the building. If taken too far, this could result in a stuffier feeling of the indoor air. Additionally, laboratory buildings must maintain incoming air for chemical fume hoods to operate properly. Thus, there is a balance we are trying to maintain between fresh air and filtration.

Generally speaking, indoor air PM2.5 concentration tends to be lower than outdoor air during periods of high wildfire smoke. Most of the University’s newer buildings have excellent filtration and do provide a refuge from the smoke.

It is best to minimize time outside during periods of elevated wildfire smoke. When you have to be outside, one option would be to wear an N-95 or KN-95 mask. These do provide filtration of smoke particles, but may not appropriate for everyone to wear.

Simple cloth or surgical masks will not provide removal of smoke particles because they don’t seal tightly, and air can pass around the mask.