Oxygen

The substance oxygen exists in two gaseous forms: (1) dioxygen, which is the oxygen we breathe, and (2) trioxygen, which is also known as ozone. In contrast to the colorless and odorless dioxygen we breathe, ozone has a light-blue color and its pungent odor is that associated with thunderstorms; lightning discharges produce ozone.

Naturally occuring ozone present in the upper atmosphere (the "ozone layer") screens out 95-98% of the ultraviolet light that comes from the sun. Excessive exposure to ultraviolet light causes sunburn and can contribute to the development of certain types of skin cancer. The presence of ozone in the upper atmosphere is a very desirable situation.

Ozone, under certain conditions, can also be found in the lower atmosphere. Here, its presence is considered undesirable. In the lower atmosphere, ozone is produced during photochemical smog formation. As the "active ingredient" in such smog, it is damaging to plants and to the human lungs.

[source: Chemical Principles by H. Stephen Stoker]