This Instructable explains how to use a gas sensor with your Arduino.This lets your Arduino smell (and hence you program responses to) overall gas levels for a variety of nasties, including ethanol, methane, formaldehyde, and a bunch of other volatile organic compounds.My cost to make this actual device was under $100, including the full Arduino kit. Here's a video:And no, I won't put a shirt on:-)I've got kits / finished versions of this and some other projects for sale @ my website The closest comparable commercial products I can find are:I learned about this after hearing about some guys who added VOC sensors to toy dogs. Not sure where/if the docs on that project are, but here's the guide that I followed. Links explaining what VOCs are and why you might want to care: information on sick building syndrome: 'A 1984 World Health Organization Committee report suggested that up to 30 percent of new and remodeled buildings worldwide may be the subject of excessive complaints related to indoor air quality (IAQ).' The Inside Story: A Guide to Indoor Air Quality: "For pollutants other than radon, measurements are most appropriate when there are either health symptoms or signs of poor ventilation and specific sources or pollutants have been identified as possible causes of indoor air quality problems. Testing for many pollutants can be expensive. Before monitoring your home for pollutants besides radon, consult your state or local health department or professionals who have experience in solving indoor air quality problems in non-industrial buildings."