Ozone
Ozone is a highly reactive gas that irritates the airways. Indoors it mostly enters with outdoor air on sunny, polluted days, and can be produced by some office equipment and air purifiers. Because it reacts readily with other compounds, even modest levels are worth tracking.
Reactive, irritating, and partly home-grown.
Ozone harms airways directly and reacts with indoor chemicals to form new pollutants — so keeping it low matters on two fronts.
Airway irritation
Ozone inflames the airways, worsens asthma and reduces lung function — effects that appear at relatively low indoor levels.
Secondary pollutants
It reacts with VOCs and other compounds to create fine particles and irritants, compounding the indoor chemical load.
Equipment sources
Laser printers, copiers and some “ionising” air cleaners can generate ozone indoors — worth catching with continuous data.
Compliance
WHO and WELL set ozone limits. Monitoring shows when outdoor smog or local equipment pushes you toward them.
Low is the goal; the limit is a ceiling, not a target.
The WHO 8-hour guideline is 100 µg/m³. Healthy interiors stay well under it; rising values usually track outdoor smog or a local source.
Mostly from outside — sometimes from inside.
Ozone is rarely emitted directly by people; it arrives with outdoor air or is generated by specific equipment.
Outdoor smog
Sunlight reacting with traffic pollution forms ozone outdoors, which enters with ventilation and infiltration on hot days.
Office equipment
Laser printers and photocopiers can emit ozone, especially older units in enclosed rooms.
Air cleaners
Some “ionising” or ozone-generating purifiers add ozone to the air — usually doing more harm than good.
Outdoor-air strategy
When outdoor ozone is high, drawing in lots of unfiltered fresh air raises indoor levels; filtration helps.