Can ozone also be found indoors?

As already mentioned, ozone is a very unstable and reactive compound. If it enters a room via outdoor air, it will react with all material that it touches. The ozone concentration will  hereby approximately decrease by half. Because the highest ozone concentrations (outdoors) are measured between 12am and 10pm, it is best to ventilate rooms during ozone smog episodes before 12am or after 10pm.

In badly ventilated rooms where a lot of (old) copying machines and/or laser printers are active, drastically increased ozone concentrations can be observed. The last generation of laser printers and copiers are equipped with ozon filters, which break down ozone and limit the leftover emission to a minimum.

In the royal decree of the 11th of April 1995 (BS. 14/6/1995, blz 17055), a maximum exposure value of 200 µg/m³ in a working area is provided for ozone.