You come home and the air is heavy. It is worst in the morning — your head is thick and your throat is dry. The children are coughing, and the windows are misting up. Poor indoor air quality is invisible, but it affects you every day — and it is far more widespread than most people realise.
Here are the causes and what you can do.
What makes air poor quality?
Indoor air is polluted by everything in the home — and everyone living in it:
CO₂ from occupants
People exhale CO₂. In a closed room the concentration rises quickly:
- Below 800 ppm: Good air quality
- 800–1,200 ppm: Acceptable, but not optimal
- Above 1,500 ppm: Headaches, fatigue, reduced concentration
- Above 2,500 ppm: Marked impact on cognitive function
A bedroom with two adults and a closed door typically reaches 2,000–3,000 ppm overnight, according to BUILD/SBi. This is the primary reason many people wake up feeling tired.
Moisture and mould
Humidity above 60% creates conditions for mould. Moisture sources include: bathroom, cooking, drying clothes, and occupants themselves (a person releases 0.5–1 litre of water per day through breathing and perspiration). Without adequate ventilation, moisture accumulates.
Particles and chemicals
- Cooking: Deep-frying and frying release particles that put strain on the airways. An extractor hood with ducting (not recirculation) is the single most important measure.
- Candles: One of the most underrated sources of pollution. A single candle emits fine particles equivalent to a busy road.
- New furniture and floors: Off-gas formaldehyde, VOCs and other chemicals in the first months after installation.
- Cleaning products: Chlorine-based products and aerosol sprays pollute indoor air.
Radon
A naturally occurring radioactive gas that seeps up from the ground. Invisible and odourless. The Danish Health Authority recommends a maximum of 100 Bq/m³ — but many homes with a basement or a ground slab without a membrane exceed this. Radon is the second leading cause of lung cancer after smoking.
The consequences
Poor indoor air quality in the home contributes to:
- Asthma and allergies — worsened by moisture, mould, dust mites and particles
- Respiratory infections — more frequent in children in poorly ventilated homes
- Headaches and fatigue — a direct consequence of high CO₂
- Reduced concentration — relevant for working from home and children’s schoolwork
- Sleep problems — high CO₂ at night disrupts sleep quality
According to WHO, poor indoor air quality is a contributing factor in 4% of the global burden of disease.
What you can do — step by step
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Measure the air. A CO₂ meter with hygrometer (300–800 DKK) gives you facts. Place it in the bedroom and living room. Measure over a week.
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Ventilate — systematically. 5–10 minutes of cross-ventilation 3–4 times a day is more effective than a window left slightly ajar. Cross-ventilation exchanges the air quickly. A window on the latch provides slow air exchange and large heat loss.
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Use an extractor hood with ducting. A recirculating extractor hood only removes fat — not moisture and particles. Ducting to the outside is far more effective. Cost of new extractor hood with ducting: 5,000–15,000 DKK.
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Reduce candles. Use LED lighting as an alternative, or limit use and ventilate afterwards.
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Install trickle vents. Vents in windows or the exterior wall (500–2,000 DKK each) provide background ventilation without opening a window.
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Install mechanical ventilation. Decentralised units with heat recovery (8,000–15,000 DKK each) are the most effective solution in existing homes. Installed in an exterior wall, requiring a 160 mm core. Central system with ducts: 40,000–80,000 DKK.
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Measure radon. A radon measurement over 2–3 months (dosimeter: 300–500 DKK) reveals whether the level is too high. Solutions: radon suction below the floor (10,000–30,000 DKK) or improved ventilation.
Which homes are most at risk?
Well-sealed houses without mechanical ventilation. Retrofitted houses with new, draught-proof windows and no ventilation system. Better insulation + tighter house = worse air, unless ventilation keeps pace.
Flats with single-aspect daylighting. No possibility of cross-ventilation. Often combined with many occupants in a small area.
Houses with gas hobs. Gas combustion releases NO₂, CO and particles directly into the kitchen. An extractor hood with ducting is critical.
Holiday homes used in winter. Closed, well-sealed, heated — but no ventilation. Moisture from occupants condenses, and air quality deteriorates rapidly.
Ventilation and heating systems
Mechanical ventilation with heat recovery (MVHR) recovers 80–90% of the heat from the outgoing air. This means fresh air with negligible heating cost. In houses with a good building envelope (insulation, draught-proof windows), MVHR is almost a prerequisite for good indoor air quality.
Children and air quality
Children are particularly vulnerable to poor air — they breathe faster and have developing lungs. According to The Danish Health Authority, children in homes with poor indoor air quality have a 30–50% higher risk of developing asthma.
Especially important in the children’s room:
- Hard floors instead of carpets (reduces dust mites)
- Wash bedding at 60°C weekly
- Trickle vent or window slightly open at night
- No candles or incense
- CO₂ below 1,000 ppm — measure with a CO₂ meter
A decentralised ventilation unit in the children’s room (8,000–15,000 DKK) is one of the best health investments you can make in your home.
How to move forward
Start by measuring. A CO₂ meter gives you answers within a week. If the numbers are too high, the next step is to improve ventilation — either with simple measures (ventilation, trickle vents) or with mechanical ventilation.
If your children are frequently ill, poor air quality may be a contributing cause — read more in children always sick: check the indoor air quality. And if you notice mould on window frames or behind furniture, it is a sign that humidity in the air is too high. If you sleep poorly, poor air in the bedroom may also be the cause.
Sources: BUILD/SBi — indoor climate, Danish Health Authority, WHO — indoor air quality