Ozone
Air quality Ground-level ozone O3
Ozone is a gas made of three oxygen atoms. High in the atmosphere it protects against UV radiation; near the ground it can harm health.
The same gas with a different job
Ozone (O₃) has three oxygen atoms, while ordinary oxygen has two. The stratospheric ozone layer absorbs some ultraviolet radiation. Ozone near the ground, however, can irritate the airways. Saying simply “ozone is good” or “ozone is bad” leaves out the crucial height.
How it forms
Ground-level ozone is generally not emitted directly. Sunlight drives reactions involving precursors such as nitrogen oxides and volatile organic compounds. Prolonged sunny, warm weather can favour its formation.
Why a park may not have the lowest reading
Freshly emitted nitric oxide can remove ozone. Ozone beside a road may therefore sometimes be lower than in the surrounding countryside, despite higher levels of other pollutants. Low ozone alone is not a certificate of clean air.
Reading the display
Values commonly use µg/m³. The averaging period matters: an hourly value differs from an eight-hour mean. A model forecast represents a wider area. When levels rise, official measurements and their guidance help, particularly for people with sensitive airways. A pleasant breeze and blue sky do not reveal the concentration.