How the flower moves

What the wilting flower is telling you about the air in the room, how its positions were chosen, and the questions teachers and students ask most.

The detailed steps below are in English for now. Translations are on the way.

The SchoolAir flower is an air-quality display with no screen and no numbers. When the air in the room is good, the flower stands up. As the air gets worse, it droops, and when a guideline has been crossed it hangs horizontal or lower. It is driven by the same sensor that feeds your school's dashboard, so the flower and the chart always agree. This page explains what "good" and "worse" mean in that sentence, because the flower is only as honest as the rules behind it.

The two things the flower watches

The sensor in the pot measures several things every five minutes: temperature, humidity, carbon dioxide (CO₂) and fine dust in several sizes. The flower responds to only two of them.

Temperature and humidity are shown on the dashboard but deliberately left out of the flower. A flower drooping because the room is warm would send the wrong message about the air.

CO₂ as a ventilation proxy

This is the idea the whole flower rests on, so it is worth being precise. CO₂ at the levels found in classrooms is not itself dangerous. You would need concentrations many times higher than any classroom reaches before the gas did you harm. So why watch it?

Because people are the only significant source of CO₂ indoors, and the only thing that removes it is fresh air coming in. That makes the CO₂ level a direct readout of the balance between how many people are breathing in a room and how much outside air is replacing what they breathe out. When CO₂ is high, everything else people exhale is high too: moisture, odours, and the fine droplets that carry colds, flu and other infections from one person to the next. Studies in schools also find that concentration and attention fall as CO₂ rises through the range a closed classroom reaches in a single lesson.

One way to picture the numbers: at 1000 ppm, roughly one and a half percent of every breath you take is air that has already been through someone else's lungs. At 2000 ppm it is nearly four percent. Nobody would drink from a glass that had been that thoroughly shared.

So the flower does not say "the CO₂ will hurt you". It says "this room is not getting enough fresh air for the people in it", which is a message you can act on in ten seconds by opening a window or a door.

How the positions were chosen

The flower has one axis of movement, from erect to fully wilted, and we had to decide what each point along it means. We anchored it to the guideline lines that already appear on your dashboard's charts, so that the flower at horizontal means the guideline line on the chart has just been crossed. A teacher who sees the stem at nine o'clock knows the same thing the chart knows, without reading a number.

CO₂

The WHO publishes no CO₂ guideline, because CO₂ is a proxy rather than a pollutant. The value used across the field for an occupied classroom is 1000 ppm, and it is the value on SchoolAir's own guideline page. Your dashboard also warns at 800 ppm, which is where the flower begins to lean.

PM2.5

Here the WHO does give guidelines, revised in 2021: an annual average of 5 µg/m³ and a daily average of 15 µg/m³. Above the daily line the WHO also publishes "interim targets" at 25, 37.5, 50 and 75, which are the rungs countries with polluted air are asked to climb down. Those numbers gave us the flower's rungs.

Putting the two together

Between the rows of each table the flower moves in a straight line, so 900 ppm sits halfway between leaning and horizontal. When the two measurements disagree, the flower follows the worse one. A room with perfectly clean dust and 1800 ppm of CO₂ looks wilted, not half-way, because averaging would hide a real problem behind a good number. The dashboard tells you which of the two is responsible.

The flower checks the latest reading every five minutes and moves only when the answer has changed by a noticeable amount, so it drifts over a lesson rather than twitching. A full journey from erect to wilted takes about a minute of movement.

Frequently asked questions

Is the air always unhealthy when the flower is wilted?

No. A wilted flower means one of the two measurements has passed its guideline, and the two mean different things. If it is CO₂, the air is stale rather than toxic: nobody is being poisoned, but the room is sharing more breath than it should, infections spread more easily, and people get drowsy and lose focus. If it is PM2.5, there is genuinely more fine dust in the air than the WHO considers safe over a day, and the sooner it is dealt with the better. The flower asks you to look; the dashboard tells you which it is.

Is it always good to open a window when the flower is wilted?

Usually, but not always, and this is the most important thing to teach. If CO₂ is the cause, opening a window is exactly right and the flower will begin to stand up within minutes. If PM2.5 is the cause and the dust is coming from outside, such as morning traffic, a nearby fire, or a smoggy day, opening the window makes the room worse, not better. In that case keep windows closed during the worst of it, run a filter if you have one, and ventilate when the outside air clears. When in doubt, glance at the dashboard: a CO₂ problem is fixed by fresh air, a PM2.5 problem depends on where the dust is coming from.

Can the flower ever reach the worst position in a normal classroom?

For CO₂, easily. A normal classroom with the door and windows closed will pass 1500 ppm within an hour of the lesson starting, and 2000 or more is common by the end of a double period. Most classrooms that have never measured CO₂ are surprised by this. For PM2.5, it is less common indoors in most of Europe. Levels above 25 µg/m³ inside a school usually mean something specific: a polluted day outside with windows open, smoke, dust from building work, or cooking close by.

How quickly does a closed classroom wilt the flower?

Faster than most people expect. Take a typical room of about 170 cubic metres with 25 students and a teacher. Each person adds roughly a third of a litre of CO₂ a minute. With the windows and door closed and only the leakage of an ordinary building, the CO₂ level climbs by something like 40 ppm every minute at the start of the lesson. Starting near outdoor air, the flower begins to lean after about ten minutes, reaches horizontal at 1000 ppm within fifteen to thirty minutes, and is fully wilted well inside the hour. Because the sensor reports every five minutes, the flower lags the room by a few minutes. The reverse is quicker: with two windows open, a room can drop several hundred ppm in five to ten minutes, and the flower rises with it.

Why does the flower ignore temperature and humidity?

Because they are about comfort, not the healthiness of the air, and mixing them in would make the flower's message ambiguous. A warm room with fresh air is fine; a cool room with 2000 ppm of CO₂ is not. Both numbers are on the dashboard if you want them.

The flower was wilted when we arrived in the morning. Why?

Three usual reasons. The room was used late the day before and has not been aired since, so the CO₂ is still high; that is real, and the fix is to open up for ten minutes. Or outdoor PM2.5 is high this morning, which the dashboard will show. Or, rarely, the sensor has had a fault and the flower is showing the last reading it trusted. If the dashboard shows no fresh reading for the room, that is the third case.

Why does the erect flower have a slight curve at the top?

Because a real flower does. The stem is a soft tube with a stiff rod inside; at the top position the rod supports it almost to the head, and the last few centimetres bend under the head's weight. It was chosen on the bench because it looks alive rather than like a ruler.

Can we change where the positions are?

Yes. The two tables live in a small settings file on the flower's own computer, and the guideline numbers can be changed without touching the mechanism. If a school wants the flower to droop at 800 ppm rather than 1000, for instance, it is one number. A class studying air quality could reasonably argue for a different table and try it.

Related: Build the flower indicator · Guideline values and where they come from · Your school's dashboard