Chlorine gas is pretty nasty stuff! It works by reacting with the water/moisture in your lungs to form hydrochloric acid (HCl). As you know, HCl is one of the 7 strong acids, which means it completely dissociates into H+ and Cl- (H30+ and Cl-, really) in water. Wikipedia says, "Chlorine is detectable in concentrations of as low as 1 ppm. Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. About 1000 ppm can be fatal after a few deep breaths of the gas." PPM stands for parts per million.

To put that in perspective, oxygen, which comprises about 20% of the earth's atmosphere is 209,460 ppm. That means if the earth's atmosphere were mere 0.095% chlorine, you and I wouldn't be talking...