Smelling Contaminants

Humans can smell H₂S, the gas that makes sewage stink, at 0.00047 ppm. CO₂, the main greenhouse gas, humans cannot smell it at all, even though as of 2012-10 its concentration was 391 ppm. In 1955 it was only 310 ppm. It is rising faster than at any time in the history of the earth.

~ Roedy (1948-02-04 age:69)