Understanding Pollutants
Understanding Temperature
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11 mins
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Published on
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Reviewed by the BREATHE editorial team
Quick Answer
Indoor temperature is one of the most direct drivers of comfort, and it also interacts with several other aspects of air quality: warmer air holds more moisture, increases off-gassing from furniture and materials, and can make CO₂ buildup feel more noticeable. There's no single universal “ideal” temperature, but widely used guidance suggests keeping living spaces at roughly 18–24°C, with 18°C as a WHO-recommended minimum for general health in cold weather and 20°C for vulnerable groups such as infants and older adults. Bedrooms are typically most comfortable slightly cooler, around 16–19°C for sleep. Continuous monitoring makes it easier to spot rooms that run consistently too warm or too cold, rather than relying on how a room feels at any one moment.
TL;DR
- What it is: A measure of how warm or cool the air is in a space, one of the most direct drivers of comfort and a factor that interacts with humidity and other pollutants.
- Why it matters: Both cold and overheated homes are linked to measurable health risks, and temperature affects sleep, concentration and general wellbeing.
- Main factors: Heating and cooling systems, insulation, solar gain through windows, occupancy, and outdoor weather.
- How to interpret them: Compare readings against WHO and comfort guidance, and watch for rooms that consistently run too hot or too cold rather than single moments.
- What to do: Adjust heating, cooling and ventilation to keep rooms within a comfortable range, and pay particular attention to bedrooms and rooms used by vulnerable people.
What is Temperature?
Indoor temperature measures how warm or cool the air is in a space, usually expressed in degrees Celsius or Fahrenheit. It's one of the most immediately noticeable indicators of comfort, and unlike pollutants such as CO₂ or PM2.5, most people can sense meaningful temperature changes without any equipment at all.
Temperature also interacts closely with other measurements. Warmer air can hold more moisture, which affects relative humidity readings, and warmer conditions generally increase the rate at which furniture, building materials and other sources release VOCs and formaldehyde. Because of these interactions, temperature is rarely considered in isolation from humidity when assessing overall indoor comfort.
Where does it come from?
Indoor temperature is shaped by a combination of heating and cooling systems, insulation and building construction, solar gain through windows, occupancy, and the outdoor climate. A room with large south-facing windows can run noticeably warmer on a sunny day than an equivalent room elsewhere in the same home, even with identical heating settings.
Appliances and activities also contribute: ovens, tumble dryers, and even a room full of people can raise the temperature of a space measurably, particularly in smaller or poorly ventilated rooms. Poor insulation and draughts have the opposite effect, allowing heat to escape quickly in cold weather and making a room harder to keep warm.
| Category | Examples |
|---|---|
| Natural sources | Outdoor weather and season, solar gain through windows, natural ventilation and draughts |
| Human-made (anthropogenic) sources | Heating and cooling systems, insulation and building construction, appliances (ovens, dryers), occupancy and activity levels |
Health and comfort effects
Temperature affects health and comfort in both directions. Homes that are too cold are linked to increased risk of respiratory infections, cardiovascular strain, and, in more severe or prolonged cases, hypothermia, particularly for infants, older adults and people with chronic health conditions. The World Health Organization estimates that cold housing contributes to tens of thousands of excess winter deaths each year across parts of Europe alone.
Overheating carries its own risks. Homes that run too warm, particularly during heatwaves, can contribute to heat exhaustion, dehydration and worsened outcomes for people with cardiovascular or respiratory conditions. Temperature also has a well-documented effect on sleep: bedrooms that run too warm can disrupt the body's natural overnight temperature decline, a key part of how sleep is regulated, while a moderately cool bedroom generally supports deeper, more restorative sleep.
Understanding measurements
Temperature is reported in degrees Celsius (°C) or Fahrenheit (°F). Comfort depends on more than the number alone, including humidity, clothing and activity level, but the following bands are a useful general reference for living spaces.
| Temperature | Interpretation |
|---|---|
| Below 16°C | Cold; increasing risk of respiratory and cardiovascular strain with prolonged exposure, particularly for vulnerable groups |
| 16–18°C | Below the WHO's general recommended minimum (18°C); acceptable briefly but not ideal for sustained occupancy |
| 18–21°C | Within WHO guidance and a widely used comfortable range for general living spaces |
| 21–24°C | Comfortable for most people; within common thermal comfort guidance for lightly clothed, sedentary activity |
| 24–26°C | Warm; generally still comfortable but approaching common overheating thresholds for bedrooms |
| Above 26°C | Commonly used as an overheating threshold for bedrooms and living spaces in UK residential guidance |
These figures are general guidance for living areas; bedrooms are generally most comfortable a little cooler, often cited as roughly 16 to 19°C, since a cooler environment supports the body's natural overnight temperature decline during sleep.
How to reduce levels
A short list of practical steps helps keep temperature in a comfortable, healthy range:
1. Set heating to maintain at least 18°C in main living areas during cold weather, and 20°C in rooms used by infants, older adults or anyone with a chronic health condition.
2. Use curtains, blinds or external shading to reduce solar gain in rooms that overheat during sunny weather.
3. Ventilate in the evening or overnight during hot weather to bring in cooler outdoor air, then close windows and blinds during the hottest part of the day.
4. Keep bedrooms slightly cooler than living areas, generally in the 16–19°C range, to support better sleep.
5. Improve insulation and draught-proofing where practical to make a home easier and more efficient to keep at a stable, comfortable temperature.
6. Use a fan or portable cooling during heatwaves for rooms without air conditioning, focusing on rooms used by vulnerable occupants first.
| Source | Impact | Recommended action |
|---|---|---|
| Poor insulation or draughts | Rooms cool quickly and are harder to keep warm | Improve insulation and draught-proofing |
| South-facing windows without shading | Rooms overheat on sunny days | Add curtains, blinds or external shading |
| Undersized or poorly maintained heating | Rooms stay below a comfortable or healthy minimum | Service heating systems and check thermostat settings |
| Heatwave conditions | Sustained overheating, particularly in top-floor or west-facing rooms | Ventilate overnight, shade windows during the day, use fans |
| Ovens, dryers and other appliances | Localised, temporary temperature increases | Ventilate kitchens and utility rooms during and after use |
| High occupancy in a small room | Gradual temperature rise alongside CO₂ | Ventilate more frequently when a room is fully occupied |
Monitoring and sensors
Most consumer and professional temperature sensors use a thermistor or a similar solid-state sensor, which measures temperature through a predictable change in electrical resistance as the material heats or cools. These sensors are inexpensive, accurate to within a fraction of a degree in most consumer devices, and require little to no calibration over their working life.
Because temperature can vary noticeably within a single room, for example near a window, radiator or in direct sunlight, sensor placement matters. A reading taken in a typical, centrally located spot in a room will usually be more representative of overall comfort than one taken right next to a heat source or a draughty window.
| Method | How it works | Best for |
|---|---|---|
| Thermistor | Measures temperature through a predictable change in electrical resistance | Compact, accurate consumer and professional monitoring |
| Resistance temperature detector (RTD) | Uses a metal element, often platinum, whose resistance changes very predictably with temperature | High-precision industrial and laboratory measurement |
| Thermocouple | Generates a small voltage from the junction of two different metals, proportional to temperature | Wide-range industrial and appliance applications |
| Infrared (non-contact) thermometer | Measures the infrared radiation emitted by a surface to estimate its temperature | Quick spot checks of surface temperature rather than air temperature |
Common myths
Myth: Warmer is always more comfortable. Reality: sustained overheating, particularly in bedrooms, disrupts sleep and can pose health risks during hot weather; a moderately cool environment is generally more comfortable and healthier than a warm one.
Myth: Turning the heating up higher heats a room faster. Reality: a thermostat controls the target temperature, not the speed of heating; setting it higher than needed doesn't heat the room any faster and simply risks overheating once that temperature is reached.
Myth: If a room feels comfortable, the temperature must be fine for everyone. Reality: comfort varies by age, activity level, clothing and health status; a temperature that feels fine to a healthy adult may be too cold for an infant or too warm for someone with a cardiovascular condition.
Myth: Air conditioning is only about comfort, not health. Reality: during heatwaves, cooling a home can meaningfully reduce the risk of heat-related illness, particularly for older adults and people with chronic health conditions.
Myth: Indoor temperature doesn't affect air quality. Reality: temperature influences humidity, and it also affects the rate at which furniture and materials release VOCs and formaldehyde, so it's closely linked to several other measurements.
BREATHE Recommendation:
Temperature can vary significantly between rooms and across the day, which makes an occasional check-in an incomplete picture. Continuous monitoring shows how each room in your home actually behaves, including which ones run too warm, too cold, or fluctuate the most. The BREATHE Airmonitor Plus tracks temperature alongside humidity, CO₂, PM1, PM2.5, PM10, TVOCs and formaldehyde, so you can see the full picture of comfort and air quality together.
Frequently asked questions
What is a healthy indoor temperature?
The WHO recommends a minimum of 18°C for general health in cold weather, rising to 20°C for vulnerable groups such as infants, older adults and people with chronic health conditions. Most comfort guidance places a comfortable range for general living spaces at roughly 18–24°C.
What is the best temperature for sleep?
Bedrooms are generally most comfortable slightly cooler than living areas, often cited as roughly 16–19°C, since a cooler environment supports the body's natural overnight temperature decline.
What temperature is considered too hot indoors?
UK residential overheating guidance commonly uses 26°C as a threshold for bedrooms and living spaces, above which sustained exposure is considered uncomfortable and, during heatwaves, a health risk.
What temperature is considered too cold indoors?
The WHO's recommended minimum for general health is 18°C, with sustained temperatures below roughly 16°C associated with increased respiratory and cardiovascular risk, particularly for vulnerable people.
Does temperature affect other air quality readings?
Yes. Warmer air affects relative humidity and tends to increase the rate at which VOCs and formaldehyde are released from furniture and building materials.
How accurate are consumer temperature sensors?
Most use a thermistor, which is generally accurate to within a fraction of a degree and requires little calibration, though placement within a room affects how representative a reading is.
Where should I place a temperature sensor for the
most accurate reading?
A central location away from direct sunlight, radiators, draughty windows and appliances will generally give the most representative reading of overall room comfort.
Does humidity affect how temperature feels?
Yes. Higher humidity makes warm temperatures feel hotter and more uncomfortable, while very dry air can make a room feel cooler than the actual temperature.
Should bedrooms and living rooms be kept at the same
temperature?
Not necessarily. Many people find bedrooms more comfortable a few degrees cooler than living areas, particularly for sleep.
Should I monitor temperature continuously?
It's useful, particularly for
identifying rooms that consistently run too hot or too cold, or for confirming
whether heating, cooling or insulation changes are actually working.
Sources:World Health Organization. WHO Housing and Health Guidelines. Geneva: WHO Press, 2018.
ASHRAE Standard 55-2023: Thermal Environmental Conditions for Human Occupancy. Atlanta: ASHRAE, 2023.
CIBSE TM59: Design Methodology for the Assessment of Overheating Risk in Homes. London: Chartered Institution of Building Services Engineers, 2017.
Okamoto-Mizuno, K. and Mizuno, K. (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(14).
At a Glance
- Indoor temperature affects comfort directly and influences humidity, off-gassing and perceived air quality.
- The WHO recommends a minimum of 18°C for general health, rising to 20°C for vulnerable groups.
- Bedrooms are typically most comfortable slightly cooler than living spaces, supporting better sleep.
- Both cold and overheated homes carry measurable health risks, particularly for the very young and older adults.
- Continuous monitoring reveals which rooms and times of day run too hot or too cold.
