Every packet in your kitchen carries a label. The air you breathe about twenty thousand times a day carries nothing. So here is one.
Fine particles, PM2.5, in micrograms per cubic metre · ten places with an instrument on the ground · best to worst by three-week average · hourly
Ordered by three-week average, cleanest first, so the readings below run out of order. Read that average with two caveats: three weeks is not a year, and these are South African winter weeks, which are the worst of it.
The order of this list is the three-week average, cleanest at the top, not the reading of the moment. Ranking by the live number would put whichever place is having a quiet hour at the top and call it the best, which is why the current readings below run out of order. The three-week average is the number that matters most to a person who lives somewhere, because harm from these particles comes from years of them rather than from any one morning. It is measured against the WHO annual guideline of 5, which is far lower than the daily one for exactly that reason. Read it with two caveats: three weeks is not a year, and these are South African winter weeks, which are the worst of it.
The colour of each row is the WHO 24-hour band that reading falls in, from meeting the guideline through the four interim targets the WHO sets for countries still working towards it. Those bands are the 24-hour ones. The simulator on this site uses the separate annual scale, whose numbers are lower, and the two are never mixed.
PM2.5 is particles smaller than 2.5 micrometres across, roughly a thirtieth the width of a human hair. Soot, smoke and the products of burning things, fine enough to carry deep into the lungs instead of being caught in the nose and throat. It is the air pollutant most closely linked to shortened lives, which is why it is the one counted here.
These are readings from sensors on the ground, not our estimate of anywhere else. Each reading covers about an hour, while the WHO figure is a ceiling for a whole day, so the percentage answers a narrower question: what the day would look like if every hour of it looked like this one. Each place is compared only with its own last three weeks, and only with readings from the same named station, because the instrument answering for a place can change when one falls silent and a different unit in a different suburb is not that place's past. Where a station has too little history of its own, we publish the reading and no comparison. Nothing here is to the minute either: a reading is typically about an hour old by the time you see it, and we refuse to use one that has aged past six hours. Where it says we do not know, the instrument has published nothing for six hours and we will not show you an old number dressed up as a new one.
You are what you breathe
Serving size 1 m³ · roughly two hours of resting breathing, about 2 000 breaths
Percentages are the concentration as a share of the World Health Organization 2021 guideline for short-term exposure. 100% is the level the WHO considers the ceiling, not a target.
Each row shows a pollutant as a percentage of the concentration the WHO says should not be exceeded over a short period. Neither half of this page tells you what is in the air where you are standing, unless you happen to be at one of the ten places above.
A nutritional label works because it compares what is in the packet against what a body can take. This does the same. Each row shows a pollutant as a percentage of the concentration the WHO says should not be exceeded over a short period.
The guidelines used here are the 24-hour values, and for ozone the 8-hour value. Those describe short-term exposure. The separate annual guidelines are lower, because sustained exposure does different damage, and those are what the life expectancy calculation on the simulator uses. The two are never mixed.
The concentrations in the label above are yours to set. That half of the page shows what a given mixture of air would look like, whether or not any air anywhere is currently like that. The readings at the top of the page are the opposite: real measurements, from real instruments, at the few places that have one. Neither of them tells you what is in the air where you are standing, unless you happen to be standing at one of those ten places, because in most of this country that is a measurement nobody is taking.
Sources: WHO Global Air Quality Guidelines 2021. PM2.5 15, PM10 45, nitrogen dioxide 25, sulphur dioxide 40 µg/m³ over 24 hours; ozone 100 µg/m³ over 8 hours; carbon monoxide 4 mg/m³ over 24 hours. Serving size assumes a resting ventilation rate of about 7.5 litres a minute.