Ozone is present in extremely low concentrations throughout the Earth’s atmosphere. It is concentrated in the stratosphere, where it protects us from being fried by sunlight, and accumulates near the surface where sunlight forges it into the key ingredient of smog.

Despite the name, the ozone layer is not composed entirely of ozone. Fifteen miles up, in the middle of the stratosphere where its concentration is greatest, ozone is present at only a mere 10 parts per million (ppm); merely 10 molecules out of every million are ozone. The rest are predominantly nitrogen and oxygen molecules just as here at the surface.

Ten ppm is 100 times its concentration in the atmosphere in general.

Because of their electron structure, oxygen atoms form two bonds. A molecule of oxygen gas has two atoms of oxygen glued together by two chemical bonds. The two atoms form the molecule O2.

Ozone molecules consist of three oxygen atoms in a triangular structure, with each atom sharing a single bond with each of its two neighbors.

The quantum energies of UV photons just happen to lie in the same range as the strength of chemical bonds between atoms in the chemicals of life, especially DNA.

UV radiation is subdivided into three types based on wavelength: UVA (315 to 400 nanometers), UVB (280 to 315 nm) and UVC (100 to 280 nm). A photon associated with light of shorter wavelengths carries a larger quantum of energy.

The sun emits ultraviolet radiation in all three bands, three-fourths of it as “soft” long-wave UVA. By the time it reaches the surface, 95% is UVA and 5% is UVB. No measurable UVC from solar radiation reaches the earth’s surface, because the shorter UVC wavelengths are completely absorbed by the ozone-oxygen cycle in the stratosphere.

An ozone molecule’s existence begins when UVC energy breaks apart a few oxygen molecules into two free oxygen atoms. Some of the unattached atoms react with other oxygen molecules to form ozone molecules. Collisions with an oxygen or nitrogen molecule carry off excess energy in the form of heat.

UVC is completely absorbed by these reactions in the upper half of the stratosphere and none remains below 20 miles altitude.

The ozone molecules formed by the above reaction then absorb UVB, which breaks them apart again into a molecule of oxygen gas and a free oxygen atom, once again with a third molecule that carries off the excess energy as heat. The UVB penetrates deeper so these recombination reactions occur lower in the stratosphere.

The ozone layer is very effective at screening out UVB; for radiation with a wavelength of 300 nm, the intensity at Earth’s surface is 350 million times weaker than at the top of the atmosphere

UVA photons are only slightly impeded by the ozone-oxygen cycle, so the UV rays that make it to the surface are about 95% UVA and 5% UVB. UVB causes most sunburns and skin cancers, but it is responsible for the production of vitamin D, which is an essential for human health.

The breaking of bonds in the ozone-oxygen cycle converts chemical energy into kinetic energy of molecular motion. The overall effect is to convert the UV into heat, without any net loss of ozone. This cycle keeps the ozone layer in a stable balance, heats the stratosphere from the top down, and protects DNA from damage by UVC.

Ozone near Earth’s surface is a different story. Ozone is a hazardous and corrosive chemical and pollutant. It forms when nitrogen oxides, carbon monoxide and volatile organic compounds react in the atmosphere when energized by sunlight. Motor vehicle exhaust, industrial emissions and chemical solvents are the sources of these potently reactive gases.

Ozone is a double-edged chemical sword, a toxic necessity, and ironically, we are removing it from where we need it in the upper atmosphere and adding it to where we can’t afford it in the lower atmosphere where we breathe.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.