Initial Sensor Research
Volatile Organic Compounds
AMS AS-MLV-P2
http://www.digikey.com/product-detail/en/AS-MLV-P2/AS-MLV-P2-ND/5117220
Datasheet: http://ams.com/eng/content/download/686543/1787717/348218
Cost: $19
Location: US
Dust sensor
Large particles (PM10 type for allergen detection)
Small particles (PM2.5 type for long term lung health)
Digikey
http://www.digikey.com/product-search/en?FV=fff4001e%2Cfff8004e&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
At least two of these sensors advertise that they can detect PM2.5 type stuff. The minimum detectable size seems to be around one micron for most sensors.
More links
http://item.taobao.com/item.htm?spm=a230r.1.14.1.3bGjJI&id=40868553809&ns=1&_u=925rqcfb5ac7&abbucket=9#detail
http://item.taobao.com/item.htm?spm=a230r.1.14.33.3bGjJI&id=39414932440&ns=1&_u=925rqcfb73d9&abbucket=9#detail
http://item.taobao.com/item.htm?spm=a230r.1.14.132.3bGjJI&id=39507827747&ns=1&abbucket=9#detail
http://item.taobao.com/item.htm?spm=a230r.1.14.131.3bGjJI&id=43202157247&ns=1&abbucket=9#detail
http://item.taobao.com/item.htm?spm=a230r.1.14.8.3bGjJI&id=38542610915&ns=1&abbucket=9#detail
Gas Sensors