Air-quality measurements as material

Across Europe, measuring stations sample the air every hour and report what they find to the European Environment Agency (EEA), which publishes the newest readings within a few hours.^1, 2 For an artist these numbers are a material like any other: they arrive every day, they belong to a real place, and they describe something nobody can see.

What is measured

Each reading is a concentration at one station, usually given in micrograms per cubic metre (µg/m³) and averaged over an hour.³ Two gases matter most for the works discussed here.

Nitrogen dioxide (NO₂) comes from burning. According to the EEA, transport is the main source and emits half of Europe's nitrogen oxides; road traffic releases them close to the ground in densely populated areas.⁴

Ozone (O₃) at ground level is not released directly by any source. It forms when heat and sunlight drive reactions between nitrogen oxides and volatile organic compounds.^5, 6, 2

Limits and how often they are passed

Under the EU directives, the hourly NO₂ concentration should not exceed 200 µg/m³ more than 18 times a year, and the annual mean should not exceed 40 µg/m³. The WHO's 2021 guidelines are stricter for the long term: an annual mean of 10 µg/m³ and a daily value of 25 µg/m³ on no more than three or four days a year.³

The gap shows in the EEA's figures for 2025: 0.5 % of stations were above the EU annual limit for NO₂, 87 % of them traffic stations, while 66.9 % were above the WHO annual guideline.⁴ For ozone, 22.4 % of stations were above the EU target value.⁵

Unverified data

The newest readings come out two to five hours after measurement and cover the last ten days. The countries have not validated or verified them, the EEA states, and they should not be used to check compliance with air-quality standards.¹ That is a limitation for a report. The same limitation, for a drawing, can be the subject: the reading as it stood on the day.

Two more things follow from the data. A station measures the air where it stands, not a street or a city. The hours also do not line up: nitrogen dioxide peaks near busy streets in the morning and evening rush, while ozone rises in the afternoon, so a day's highest hour of each comes from different times.²

Where to see it

Olof Broström's Unverified Air makes one drawing a day for a year, from 4 October 2026 to 3 October 2027. Each day the program draws a country at random, then a station there that measures both gases, and lays the previous day's highest hour of each over the buildings around it: nitrogen dioxide as red lines on the open ground, ozone as blue lines over the roofs, closer together as the values rise. The air-quality figures are the EEA's unverified up-to-date data, published under CC BY 4.0.²