A badge, that clips onto the strap of the bag of a public transport user.

Logs temperature, humidity, CO2 and CO.

This is augmented by GPS, accelerometer, and timestamps in the user's cellphone.

Bluetooth LE interface between Badge and phone.

All saved to a web service / cloud - The bus/route they were on is calculated. Weekly reports on a city's environment quality for public transport users - comparison by operators etc.