A new study by air quality scientists tested four models of low-cost air quality monitors during actual wildfire pollution events and found that their readings of PM2.5 – or particulate matter under 2.5 microns, which has been linked to respiratory and cardiovascular issues – were consistently higher than the reference monitor used by the regulatory agencies; however, since each monitor had a relatively consistent response to the smoke, it is possible to use the readings to estimate true PM2.5 levels.