It depends how high off the ground ozone floats to determine whether it is your friend or your enemy.
When it is adrift high up in the stratosphere above the weather systems, the molecule that is composed of three bonded oxygen atoms (O3) absorbs ultraviolet light. This colorless gas umbrella shields life below it from fatal doses of the radiation that would otherwise leave the planet’s surface barren. Ozone is a highly active substance, and compounds like human-made chlorofluorocarbons readily break it down. These have helped to thin large areas of the ozone layer. But in a rare bit of good environmental news, decades of efforts to let the thinning repair itself seem to be succeeding.
Ozone also exists near the ground, the result of sunlight hitting airborne emissions from vehicles, power plants and industrial activity. This low-level ozone is the main component of smog and can harm animal and plant tissues. According to the Environmental Protection Agency, the amount of ozone near the ground, the bad stuff, decreased by 25 percent across the U.S. between 1980 and 2012. Looking at the most recent period, the improving trend decreased to a more modest 9 percent reduction in ozone concentration nationally between 2000 and 2012. Much of this reduction can be attributed to better technologies and controls that have scrubbed ozone precursors from emissions.
National averages, though, can gloss over deleterious ozone spikes in places like Houston and L.A., two of America’s most well known smoggy cities, as well as in rural locales. In fact, NASA researchers say ozone concentrations are rising globally with increasing industrial output, heavier land use and wealthier societies in countries like China, India and Brazil. Elevated ozone levels are estimated to increase global mortality by 152,000 people per year.