Most air purifiers (not just smart ones) have sensors that measure the density of various airborne pollutants such as nitrogen dioxide (NO2, a toxic gas from car exhausts and power plants), PM2.5 (particulate matter smaller than 2.5 microns in size, including industrial emissions and tobacco smoke), PM10 (particulate matter up to 10 microns, including dust, mold, and pollen), and VOC (volatile organic compounds, or potentially harmful gases from things like cleaning products and paint).  Smart air purifiers typically feature Wi-Fi radios that let them connect to a companion app that shows your home's air quality. Some apps offer graphs of your air quality over time. Dyson breaks it down by pollutant, showing charts for each one.  Most smart air purifiers feature an auto mode, which automatically adjusts the fan speed based on the level of pollution detected. They also typically let you set up an air purifying schedule, so the machine automatically turns on and off at specific times. Some models feature Alexa, Google Assistant, and Siri support, so you can control them with voice commands.  If a smart air purifier is out of your budget, consider the $69.99 Amazon Smart Air Quality Monitor, which simply tracks airborne pollution levels around the clock. You can pair it with a traditional air purifier to save a bit of money.