What is the unit ppm?

ppm stands for parts per million. For gases in air it is a volume fraction: 1 ppm means one particle of the gas per one million particles of air, which is 0.0001 percent. Fresh outdoor air contains about 420 ppm CO₂; from roughly 1,000 ppm indoor air counts as stale. ppb (parts per billion) is one thousandth of a ppm.