AirCasting is designed to record, map, and share health and environmental data using a smartphone. There are several ways to participate. The easiest is to download the AirCasting app (Android), and send back data on sound levels as recorded by the microphone in your handset.
From there, you can crank it up a notch by building your own temperature, humidity, carbon monoxide (CO) and nitrogen dioxide (NO2) gas concentrations monitor. The plans for the Arduino-powered unit can be found at this link (PDF download).
You can also buy a Bluetooth heart monitor or a BioHarness to measure your heart rate and breathing, and match that data with the local air quality to give researchers data about how air quality affects our bodies. If you really want to go all out, you can also build your own luminescent apparel to make a fashion statement while you aircast.