Indoor environment factor · measured in % (relative humidity)
Relative humidity
Also: RH, humidity, indoor humidity
The amount of water vapour in the air expressed as a percentage of the maximum the air can hold at its current temperature. It is set by ventilation, moisture sources and temperature, and it is the variable that decides whether mould has a chance to grow.
Why it matters indoors
Relative humidity describes how close the air is to saturation. At 100 % water condenses; at 20 % the air is desert-dry. Because the saturation point rises with temperature, the same air reads a different RH at different temperatures — and the cold surfaces of a room (window frames, external corners, the wall behind a wardrobe) reach saturation long before the room average does.
What drives it indoors
Moisture sources: people (breathing, perspiration), cooking, showering, drying laundry, plants, aquariums, and water ingress from leaks or rising damp. Removal: ventilation to drier outdoor air, extraction in kitchens and bathrooms, and dehumidification. Temperature: heating lowers RH without removing any water; cold walls raise local RH.
Health effects
Low humidity dries the mucous membranes, increases irritation from other pollutants and prolongs the survival of airborne viruses. High humidity supports mould, dust mites and bacteria, and is associated with asthma, allergic rhinitis and respiratory infections; the WHO concluded that occupants of damp buildings have a substantially higher risk of respiratory symptoms.
Guideline values
| Body | Value | Meaning | Source |
|---|---|---|---|
| ASHRAE 62.1 | ≤ 65 % | upper limit for occupied spaces with mechanical cooling | ASHRAE 62.1 |
| ASHRAE 55 | humidity ratio ≤ 0.012 kg water / kg dry air (≈ 60–65 % at typical temperatures) | comfort envelope | ASHRAE 55 |
| World Health Organization | no numeric limit; persistent dampness and condensation should be prevented | dampness and mould guidelines | WHO 2009 |
| Common practice | 40–60 % | balance between dryness and mould/mite risk | — |
Slovak, UAE and Hong Kong values are being compiled in the indoor air quality limits table.
How it is measured
Capacitive sensors use a polymer whose electrical capacitance changes with absorbed water. Combined with temperature, a monitor can compute dew point — the temperature at which condensation forms — and flag when any surface in the room is likely to be at or below it.
What the studies report
| Linked to | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| Indoor plants (phytoremediation) | median 29.1 | — · office | experiment with indoor plants in offices: relative humidity, indoor air temperature and carbon dioxide measured in offices with no plants, with 5 plants and with 18 plants | F-1492 Jiang et al. Effects of indoor plants on CO2 concentration, indoor air te… (2024) | machine |
| Natural ventilation | difference 3.61 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0343 Window typology as a determinant of indoor air quality in na… (2026) | machine |
| Natural ventilation | difference 1.64 | — · office | cross-sectional observational study of 54 naturally ventilated government offices in Enugu, Nigeria: CO, CO2, formaldehyde, TVOC, PM2.5 and humidity measured and regressed on window typology (casement, projecting, louvered) with architectural covariates | F-0346 Window typology as a determinant of indoor air quality in na… (2026) | machine |
| — | mean 37 | kindergarten children · kindergarten playroom | measurements of CO2, air temperature and relative humidity during occupancy in 311 classrooms and playrooms of schools and kindergartens throughout Slovenia before energy-efficiency renovation and before the COVID-19 pandemic (302 naturally and 9 mechanically ventilated spaces), with a comparison of measured averages against a COVID-19 airborne transmission model | F-0405 Measurements of air quality in kindergartens and schools in … (2022) | machine |
| — | mean 50.2 | — · classroom | comprehensive assessment of indoor air quality and thermal comfort in classrooms: carbon dioxide, operative temperature and relative humidity measured over the school year and compared with the minimum and the optimum requirements of the Spanish RITE standard and Royal Decree 486 | F-1489 A Comprehensive Assessment of Indoor Air Quality and Thermal… (2023) | machine |
| — | mean 50 | — · dwelling | wireless measurement of temperature, relative humidity and occupancy in the rooms of a dwelling over a summer period | F-1493 Szagri et al. Wireless Temperature, Relative Humidity and Occupancy Monito… (2022) | machine |
| — | other 80 | — · hospital | field study of indoor environment quality in Chinese hospitals: temperature, relative humidity and humidity ratio measured in inpatient rooms, air-conditioned offices, treatment rooms, nurse stations and corridors, with satisfaction questionnaires from patients, visitors and staff | F-1494 A field study on indoor environment quality of Chinese inpat… (2019) | machine |
| — | mean 76.5 | — · hospital | combined measurement and survey study of occupational heat stress in hospital settings: indoor air temperature and relative humidity logged in sensor boxes across wards between mid-May and mid-October of 2024 and 2025 | F-1495 Occupational heat stress in hospital settings: a combined as… (2026) | machine |
| — | mean 43.64 | — | statistical evaluation of indoor air quality parameters: temperature, relative humidity and carbon dioxide measured and compared with the values of ASHRAE, MAK, Canada and Hong Kong | F-1496 Statistical Evaluation of Indoor Air Quality Parameters in C… (2018) | machine |
| — | share-above 87 per 70 % | — · dwelling | BRANZ overview of indoor air contaminants in New Zealand homes, including a study of a 1980s Wellington house with a slab floor and single glazing in which relative humidity and carbon dioxide were logged room by room | F-1530 Ltd An overview of indoor air contaminants in New Zealand houses | machine |
| — | mean 61 | — · classroom | BRANZ research note on ventilation and air quality in New Zealand classrooms: carbon dioxide, relative humidity, particulate matter and ventilation rates measured with the SKOMOBO platform in classrooms in Wellington, Hawke's Bay, Christchurch and Dunedin, with a teacher survey | F-1538 Ltd An overview of indoor air contaminants in New Zealand school… | machine |
Each row is one Finding — a sentence as printed in one study, recorded with its page in the Coridum Knowledge Base. Follow the Finding number for the exact wording and the citation. Checked says who verified the row: machine means every automatic check passed and the surrounding text was read; person means someone read the sentence in the document. Rows are re-checked by a person over time. The studies are curated in the Coridum Knowledge Base; the reading of them together is a Claim, shown separately.
How Coridum measures it: Coridum devices use a capacitive humidity sensor paired with a temperature sensor, so that dew point and mould risk on cold surfaces can be calculated, not just the room average.
Frequently asked questions
What is the ideal indoor humidity?
Between roughly 40 % and 60 %. Below 30 % the air dries the skin, eyes and airways and viruses survive longer; above 60 % sustained, condensation on cold surfaces and mould growth become likely, and dust mites thrive.
Why does humidity rise at night in bedrooms?
Two sleeping adults exhale and perspire close to a litre of water over a night. With the door and windows closed, that moisture has nowhere to go, so relative humidity climbs — often past 70 % by morning — which is why condensation appears on windows.
Does relative humidity depend on temperature?
Yes. Warm air holds more water, so the same amount of moisture gives a lower RH in a warm room and a higher RH in a cold one — or on a cold wall. That is why mould appears in cold corners and behind wardrobes on external walls.
Sources
- WHO guidelines for indoor air quality: dampness and mould. World Health Organization Regional Office for Europe, 2009.
- ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy.
- ANSI/ASHRAE Standard 62.1: Ventilation and Acceptable Indoor Air Quality.
- The dichotomy of relative humidity on indoor air quality (2007). Environment international. doi:10.1016/j.envint.2007.04.004 CKB 0004
- Assessment of Indoor Particulate Matter and Teacher’s Perceived Indoor Climate in Government Schools of Bilaspur (2024). Indian J Occup Environ Med. doi:10.4103/ijoem.ijoem_104_23 CKB 1123
- Influence of Carbon Dioxide and pH on Influenza Virus in Sessile Saliva Droplets… (2026). Environ Sci Technol. doi:10.1021/acs.est.5c12672 CKB 1206
CKB = document number in the Coridum Knowledge Base, our curated library of peer-reviewed studies. Follow the DOI for the original paper.
Cite this page
Coridum Research. Relative humidity. Coridum Knowledge G-0005, 16 September 2026. https://coridum.com/k/G-0005/