Can an Outdoor Air Quality App Tell Me When to Open a Window?

Swiss outdoor air data, read next to cooking, indoor CO₂ and fine particles, to help you decide when to open the window.

Key takeaways

• Tap the colour. Look for the pollutant, the station or map estimate, and the period the reading covers.
• Start with what just happened indoors. Frying rösti, taking a shower and having friends round each give you a different reason to air the room.
• Compare like with like. Use PM₂.₅ in the same units over the same period when comparing outdoor and indoor particle levels.
• Let conditions outside influence the timing. If smoke is passing the window, wait for it to clear. In ordinary winter weather, a brief, wide opening works well.
• Keep one small record. A note of when the pan came off the heat and when the window opened can explain a puzzling bend in a graph.

The question at home

The rösti is crisp, the pan is off the hob, and someone reaches for the kitchen window. Someone else has their phone out. The air quality app shows amber over town. You can smell supper from the hallway. Is this a good moment to open up?

An outdoor app can help you choose a time to air the room. Use it to pick a cleaner spell outdoors, then decide how urgently the room needs airing. Tap the coloured dot to see what put it there. An amber afternoon driven by ozone is a different situation from an evening when fine smoke is drifting past the window. Meanwhile, that pan has been making particles of its own. If the outdoor particle trend is low and there’s no smoke at the window, airing the kitchen while the extractor runs is a reasonable choice. Swiss airCHeck · Indoor particle sources

What does the dot outside your house represent?

In Switzerland, the federal NABEL network and cantonal and city stations measure outdoor air. Their readings also feed the models used to colour a map between stations. On the map, your street may have its own shade all the way down to the corner. Zooming in gives you a better view of the street, but the measuring instrument stays where it was. In Bern, for example, the federal Bollwerk station represents a busy urban road. Think about how well that spot resembles the side of the building where your window opens. Swiss outdoor air maps · NABEL station descriptions

The Swiss short-term index takes whichever of three pollutants scores highest. Ozone is averaged over the previous hour, while nitrogen dioxide and PM₁₀ are averaged over the previous 24 hours. A particle index updated at supper can still reflect this morning’s traffic. Other apps may use a different index, so check the legend and whether you’re looking at a measurement or a forecast. How Switzerland calculates its index

If wood smoke passed your building just before supper, it may scarcely register in a daily average. For a particle question, open the trend for PM₂.₅, particles up to about 2.5 micrometres across, and look for a recent change. PM₁₀ includes a wider range of particle sizes. For a Swiss PM₂.₅ trace, choose a station that measures it, select “Hourly means” and display the graph. If ozone drove the colour, its advice still applies even on a low-particle day. NABEL station data

What physics allows

One window, two directions on the graph

Picture the room after everyone has sat down to eat. People are still breathing out carbon dioxide (CO₂), and particles from frying are drifting away from the hob. Opening the window brings outside air into that mixture. When the air coming in has less CO₂ than the room, it dilutes the CO₂. What happens to the particles depends on how much of that same pollutant is on either side of the window. That’s how a CO₂ reading can fall while the particle reading beside it rises. Indoor and outdoor measurements in occupied homes

A little kitchen arithmetic gives you a feel for the size of the change. Suppose the room starts at 30 micrograms of PM₂.₅ per cubic metre and the incoming air carries 10. If half the final mixture comes from outside and the air mixes evenly, you’d get 20 µg/m³. Reverse the starting values and you still get 20, only now opening the window has raised the indoor reading. Those are invented numbers for mixing. A real pan can keep giving off particles after the heat is off, and you rarely know what fraction of the room’s air has been replaced. Pan-frying experiment

Time matters here, too. A brief peak by the cooker and an elevated level that lasts all evening could produce the same number on a screen at one moment. From an exposure point of view, it matters where you spent that hour. The World Health Organization’s PM₂.₅ guideline of 15 µg/m³ refers to a 24-hour average. A one-minute reading of 16 µg/m³ on the kitchen shelf gives you far too little information to use that guideline as a signal to shut the window. WHO air quality guidelines

What tests show

Where each claim stands

• Supported: In 40 homes in Leipzig and Berlin, researchers estimated average air exchange from CO₂ measurements at 0.2 air changes per hour with windows closed and 3.7 with windows open, with considerable variation from home to home. (Zhao and colleagues)
• Supported: Repeated indoor and outdoor measurements in Swiss homes found that the ratio of indoor to outdoor particle concentrations varied widely between homes, which limits how closely an outdoor reading can predict your living-room level. (Meier and colleagues)
• Supported: In a pan-frying experiment in one US flat, opening the kitchen window lowered average PM₂.₅ in the kitchen and living room, while bedroom levels were higher in the hour after cooking than when the kitchen window had stayed shut. (Xiang and colleagues)
• Untested: Whether waiting for the app to turn green would reduce your family’s indoor particle exposure more than your usual airing routine is a question these studies did not test.

What you can do anyway

Decide what needs to leave the room

After frying, start with the cooker hood if it vents outdoors. It can catch particles while they’re still rising from the pan. Use the back rings where practical, keep the hood running while you finish cooking, and look outside before deciding whether to open the window too. If your hood recirculates air through a filter, it sends the air back into the kitchen. You still have to decide how to let steam and stale air out. US EPA guidance on cooking

After a crowded evening, an indoor CO₂ trend can help you decide when the room needs fresh air. In a home that relies on windows, the Swiss Federal Office of Public Health recommends opening them fully for at least five minutes during the heating season. In very cold weather, in wind or with cross-ventilation, it gives at least three minutes as a starting point. Wind and layout make a difference. If your home has mechanical ventilation, check how the system is set up and whether there’s a boost setting for visitors. Swiss guidance on airing and heating

After a shower, use the bathroom extract if you have one, or air the bathroom directly when outdoor conditions allow. The mist on the mirror is a clue about moisture. An indoor humidity reading alongside room temperature is more useful here than the colour on your phone. Swiss guidance on moisture and ventilation

When smoke is outside, a different routine makes sense. If a chimney plume is passing the window and the room can wait, leave it closed for now. During a wider smoke episode, follow local instructions, cut back on particle-producing activities indoors, and air the room when outdoor conditions improve. A particle filter you already own can help with smoke while the window is closed. It won’t remove the CO₂ that people keep breathing out. Smoke and indoor air

Try it at the same window on two ordinary evenings

Pick a room where you actually spend time. Before looking at the app, jot down what’s been happening. “Fried supper, hood on, kitchen door open” is a perfectly good entry. On another evening, try the same window when nobody has been cooking. If you don’t own a sensor, you can still practise reading the station name, pollutant and time period, and notice a passing plume that the map might miss. Just write down what you observed.

If you have a particle monitor, leave it in the same place on both evenings, clear of steam, the cooker and the draught from the window. Write down which window you used, the opening and closing times, and whether the hood was running. Look at the indoor curve before you opened up, while the window was open and after you closed it. Optical monitors estimate particle levels from scattered light. Water droplets can scatter light too, so a jump as steam rolls across the instrument is hard to interpret. Laboratory and field tests of an optical sensor

Now lay the outdoor PM₂.₅ record beside yours. If the station gives hourly means, compare them with indoor averages for those same hours. Keep the finer indoor trace to see whether the rise began when the pan heated up or only once you opened the window. If the app and monitor differ by just a few units, wait for a clearer pattern before changing your routine. A daily outdoor average can give the evening context, but it can’t settle what happened during those few minutes. If CO₂ falls and particles rise while the window is open, check the outdoor trend and your cooking notes before choosing a different time. Repeat on a comparable evening before drawing a conclusion.

Back at the rösti supper, you might discover that the indoor curve was climbing well before anyone touched the window. That gives you something specific to try next time. Turn the extractor on as the pan heats, keep the monitor in its usual place, and see what happens by the time you sit down to eat.

Related on SolarHealth

• CO₂ as a ventilation compass
• Air purifiers without the hype
• Breathing well at home

Sources

• Swiss Federal Office for the Environment. Current outdoor air maps, station descriptions and NABEL hourly data query.
• Cercl’Air and OSTLUFT. What airCHeck displays and how the Swiss short-term index is calculated.
• Swiss Federal Office of Public Health. Guidance on ventilation and heating at home (German).
• World Health Organization. Global air quality guidelines.
• Zhao and colleagues. Particle measurements and ventilation in 40 German homes.
• Meier and colleagues. Differences in indoor and outdoor pollutant concentrations in Swiss homes.
• Xiang and colleagues. Cooking-related PM₂.₅ under different ventilation and filtration conditions.
• Jayaratne and colleagues. How humidity and fog influence an optical particle sensor.
• US Environmental Protection Agency. Indoor particle sources and kitchen extraction and indoor air during wildfire smoke.