DIY Arduino Water Meter
labs.teague.com
labs.teague.com
I have also seen "odor sensors" on air filters. But your best bet is probably to suck air into your house through a good filter, like: http://www.air-n-water.com/product/healthproplus.html (which has a duct kit so you can put your intake outside). Then the air you are breathing is always clean.
(To avoid your readers reaching for the readability button, consider using something other than tiny white letters on light grey background.)