Yet there are places where ozone remains at high levels, including much of the West. In the US, ozone has decreased substantially in recent decades thanks to regulations governing the emission of nitrous oxide and other precursor pollutants. When these airborne pollutants meet sunlight and warm temperatures, they go through a chemical reaction and become ozone. It’s chemically similar to chlorine, but not as toxic. Ground-level ozone is created mostly by humans, formed by pollutants-such as nitrous oxide, methane, and volatile organic compounds-that are emitted by cars, trucks, refineries, power plants, and oil and gas development. It is also a greenhouse gas-the third worst after carbon dioxide and methane.
“Good up high, bad nearby” is the phrase some scientists use. At ground level, though, ozone is toxic to life. The 1987 Montreal Protocol phasing out industrial chemicals that were destroying the ozone layer is considered one of the most successful international environmental actions ever taken. Ozone-an invisible, odorless gas in the upper atmosphere-is essential for life because it shields the earth from the sun’s harmful ultraviolet rays. “It induces the most widespread damage to plants, and it’s a very serious threat to biodiversity.” In some parts of Asia, he said, ozone levels are 10 times the critical thresholds. “Ozone is the most damaging pollutant in the world,” said Evgenios Agathokleous, a professor of environmental resources at the Institute of Ecology at Nanjing University of Information Science & Technology in China and one of the top researchers in the field.
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