What is a Federal Reference Method?

Federal Reference Method (FRM) “monitors collect particulate samples on filters, which are later weighed and analyzed in a laboratory. Because of this two-step process, PM2.5 air quality data collected with FRM monitors are not immediately available. This approach is more labor intensive and provides less temporal resolution than continuous PM2.5 mass monitors,” according to the California Air Resources Board. FRMs are the gold standard for air quality reporting. Other air quality monitoring devices are often compared to FRMs to evaluate efficacy.

What is a Federal Equivalent Method?

According to the California Air Resources Board, “The US EPA designated certain models of continuous monitors as Federal Equivalent Method (FEM) monitors. Since these monitors are considered equivalent to the FRM monitors, they can be used to determine compliance with federal standards. In addition to being less labor-intensive than filter-based methods, continuous concentration monitors provide data at an hourly temporal resolution.” Essentially, FEMs are air quality monitors that use different air sampling methods than FRMs but are still considered equivalent and appropriate for federal standards.

What is a low-cost sensor?

According to the EPA, “A low-cost air pollution monitor is a device that uses one or more than one sensor and other components to detect, monitor and report on specific air pollutants like particulate matter (PM) or carbon dioxide and/or environmental factors such as temperature and humidity.” Compared to the high cost of FRMs or FEMs, low-cost sensors generally fall into the low hundreds to low thousands price range. PurpleAir sensors are considered low-cost sensors.

Low-cost sensors help fill in the gaps where FRMs or FEMs may not be able to evaluate air quality.

Learn More

Which Sensor to Choose
PurpleAir Sensors Functional Overview
What Do PurpleAir Sensors Measure and How Do They Work?