Indoor air quality: why measuring matters

We spend roughly 90% of our lives indoors – at home, at work, at school. The air in those rooms has a direct effect on how well we sleep, how clearly we think and how healthy we stay. Yet most indoor pollutants are invisible and odourless. By the time you notice "stuffy" air, levels have often been too high for a while.

A good air quality monitor turns this invisible world into clear numbers and colours. It shows you when to ventilate, what is causing a problem and whether your measures actually work. And because ventilating only when it's needed saves heating energy, measuring often pays for itself.

Below we explain the six values our sensors measure, what they mean and what you can do about them.

CO₂ – the ventilation indicator

Every time we breathe out, we add carbon dioxide (CO₂) to the room. Outdoor air contains about 420 ppm (parts per million). In a closed bedroom or a busy meeting room, levels can easily climb above 2,000 ppm within a few hours.

CO₂ itself is not toxic at these levels, but it is the best indicator of how fresh the air is. When CO₂ rises, so do other things we exhale – moisture, odours and airborne viruses. Studies have linked levels above 1,000 ppm to lower concentration, slower decision-making, headaches and fatigue.

• Below 800 ppm – excellent, well-ventilated
• 800 – 1,200 ppm – acceptable, but keep an eye on it
• 1,200 – 2,000 ppm – poor, time to ventilate
• Above 2,000 ppm – bad: open windows or increase ventilation now

Tip: Link your CO₂ sensor to your mechanical ventilation via a smart plug or your home automation system. It then only runs at high speed when it's really needed – fresh air when you need it, and no warm air blown outside when you don't.

Temperature – comfort and energy

Temperature has a big influence on comfort, sleep and productivity. Most people feel comfortable at 19–22 °C during the day; for good sleep a bedroom of around 16–18 °C is ideal. Above about 25 °C, concentration and work performance noticeably decline.

Measuring temperature per room also helps you save energy: every degree lower on the thermostat saves roughly 6% on heating. And in combination with humidity, temperature tells you whether there's a risk of condensation and mould.

Relative humidity – not too dry, not too damp

Relative humidity (RH) tells you how much moisture the air holds compared with the maximum at that temperature. The healthy range is 40 – 60%.

• Below 30–35% – dry eyes, irritated airways and skin, static electricity; many viruses survive longer in dry air
• 40 – 60% – the sweet spot for health and comfort
• Above 60–65% – dust mites thrive and the risk of mould and condensation increases, especially on cold walls and windows

Cooking, showering, drying laundry and even breathing all add moisture. In winter, short but intense ventilation (opening windows wide for 5–10 minutes) removes moisture without cooling down walls and furniture.

Particulate matter (PM1, PM2.5, PM4, PM10)

Particulate matter, or fine dust, consists of tiny particles floating in the air. The number refers to the size in micrometres: PM10 is smaller than 10 µm, PM2.5 smaller than 2.5 µm – about 30 times thinner than a human hair. The smaller the particle, the deeper it penetrates into the lungs, and the finest particles can even enter the bloodstream.

Long-term exposure to PM2.5 is linked to respiratory and cardiovascular disease. That's why the World Health Organization (WHO) tightened its guidelines in 2021: an annual average of no more than 5 µg/m³ for PM2.5 (15 µg/m³ as a 24-hour average) and 15 µg/m³ for PM10 (45 µg/m³ per 24 hours).

Indoors, the biggest sources are often surprising: frying and baking, candles, incense, wood stoves and fireplaces, smoking, and even toasters and laser printers. Outdoor sources such as traffic and wood burning in the neighbourhood also find their way inside.

Tip: Always use your extractor hood when cooking, and watch how the PM values drop – or rise – when you open a window. If wood burning is common in your street, you may sometimes be better off keeping windows closed and using mechanical ventilation with a filter.

VOC – volatile organic compounds

VOCs are a large group of gases released by everyday products: paint, varnish and glue, new furniture and flooring, cleaning products, air fresheners, perfume and cosmetics, but also cooking and people themselves. Well-known examples are formaldehyde, acetone and alcohols.

High VOC levels can cause headaches, irritated eyes and airways, nausea and tiredness. Some VOCs, such as formaldehyde and benzene, are harmful with long-term exposure.

Our sensors use the Sensirion VOC Index, which runs from 1 to 500. The index learns the normal situation in your room: 100 is the average of the past 24 hours. Values above 100 mean there are more VOCs than usual, values below 100 mean fewer. This makes it very easy to spot events – a freshly painted wall, a cleaning session or a spray can – and to see how quickly ventilation clears them.

NOx – nitrogen oxides

Nitrogen oxides (NO and NO₂) are formed during combustion. Indoors, the main source is cooking on gas, followed by gas heaters, water heaters without a flue, candles and fireplaces. Outdoors, traffic is the main source.

NO₂ irritates the airways and is associated with asthma and respiratory complaints, especially in children. The WHO recommends an annual average of no more than 10 µg/m³ for NO₂ (25 µg/m³ as a 24-hour average). Measurements in kitchens with a gas hob often show much higher peaks during cooking.

The Sensirion NOx Index also runs from 1 to 500, but here the normal situation is 1. Any higher value indicates a NOx event, such as cooking on gas – a clear signal to switch on your extractor hood or open a window.

What you can do

• Measure first – you can only improve what you can see
• Ventilate smartly – keep trickle vents open and ventilate extra when CO₂, humidity or VOC values rise
• Automate – let your sensor switch your ventilation via a smart plug, Home Assistant, Homey or Domoticz
• Use your extractor hood – every time you cook, and especially when cooking on gas
• Limit sources – fewer candles and air fresheners, low-emission paints and products
• Keep an eye on the bedroom – you spend a third of your life there, often with the door and windows closed

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