Carbon monoxide

Colorless, odorless and tasteless gas that is toxic to humans in concentrations above about 35 ppm. Lighter than air. 
Density at 0 °C = 1,250 kg/m3
Density at 25 °C = 1,145 kg/m3
Lower explosive limit (LEL) = 12,5 %vol

Health exposure limits (Directive 2000/39/EC):
8 hours TWA = 20 ppm (23 mg/m3)
15 mins STEL = 100 ppm (115 mg/m3)

Carbon monoxide