Health
Indoor air quality in offices, how to measure it
An office has no single air quality number. What to measure (CO2, temperature, humidity, particles, TVOC), which source gives each reference value, and what low-cost sensors can tell you.
Indoor air quality in an office is not one number. It is a set of readings (CO2, temperature, humidity, particles, volatile organic compounds), and each has its own reference value, its own averaging time and its own measurement problems. A reading means little without knowing when and where it was taken.
For an office assessment, the practical core is CO2, temperature and relative humidity, logged continuously, because low-cost sensors for those held up best in the evaluations cited below. Particles come next. TVOC is a screening signal, and anything chemical that needs a name goes to a laboratory.
What to cover when you measure indoor air quality in an office
CO2 points to ventilation (US EPA's school reference guide calls a CO2 monitor useful for indicating when outdoor air ventilation may be inadequate) and says nothing directly about particles or chemicals. According to a REHVA article by Atze Boerstra on the CEN standard EN 16798-1:2019, its values for landscape offices in categories I, II and III are 550, 800 and 1,350 ppm of CO2 above the outdoor level. Absolute numbers are in What is a good CO2 level in an office?.
Temperature comes from the same standard. The ranges below are operative temperatures for sedentary work (1.2 met), with 1.0 clo of clothing in winter and 0.5 clo in summer. Bjarne Olesen's summary of the standard links the categories to a predicted percentage of dissatisfied people (PPD) below 6, 10 and 15 percent.
Operative temperature is a weighted average of air temperature and mean radiant temperature, so an air temperature reading is a different quantity. ISO 7730 defines PMV and PPD, the comfort indices behind these categories, and its current edition is ISO 7730:2025. ASHRAE 55-2023 covers thermal conditions for healthy adults and excludes air quality, acoustics and illumination.
The EN 16798-1 temperature ranges assume relative humidity of 40 percent in the heating season and 60 percent in the cooling season. For mold control, US EPA school guidance recommends keeping relative humidity between 30 and 60 percent, and its problem-solving tool asks about humidity above 60 percent, condensation and damp areas.
For particles, WHO's 2021 global air quality guidelines recommend 5 µg/m³ as an annual mean and 15 µg/m³ as a 24-hour mean for PM2.5, and 15 µg/m³ (annual) and 45 µg/m³ (24-hour) for PM10. The 24-hour levels are the 99th percentile of daily values, which allows three to four exceedance days a year. These are non-binding health guideline levels, and each is tied to its averaging time, so a single morning reading cannot be checked against them.
WHO says its guidelines are applicable to both outdoor and indoor environments and cover all settings where people spend time, but do not cover occupational settings. In another section it lists the workplace among indoor environments, and the text does not say whether an ordinary office counts as an occupational setting. WHO also says that in indoor environments without indoor sources, pollutants from outdoors are the main cause of indoor air pollution.
TVOC, the sum of volatile organic compounds, is a screening indicator. Germany's indoor air committee (AIR) says the TVOC value does not allow a health assessment but is a relevant indicator. Its reference value is 950 µg/m³, the 95th percentile in the 2014 to 2017 German Environmental Survey (GerES V), where the median was 270 µg/m³. Above 950 µg/m³ the AIR guide suggests more ventilation, possibly a changed cleaning routine, and a search for visible sources.
Individual substances have their own values. WHO's 2010 indoor guidelines give 0.1 mg/m³ for formaldehyde over 30 minutes and state that for benzene no safe level of exposure can be recommended. For radon WHO proposes a reference level of 100 Bq/m³ and says it should not exceed 300 Bq/m³ if 100 cannot be reached. The 2010 text names offices among indoor settings in its foreword, but its introduction says the workplace was specifically excluded. A TVOC reading is a sum and does not tell you which substances are in it.
Mold has no numeric limit. WHO's 2009 guidelines state that no quantitative health-based guideline values or thresholds can be recommended for microbial contamination, and that persistent dampness and microbial growth on interior surfaces should be avoided or minimized. US EPA's school guidance relies on looking for water stains, moldy odors and standing water, and a humidity log can show which rooms stay damp (general reasoning).
What the EU directive says
Directive (EU) 2024/1275 defines indoor environmental quality in Article 2(66) as the result of an assessment of the conditions inside a building that influence the health and wellbeing of occupants, based on parameters such as temperature, humidity, ventilation rate and contaminants. Article 13(4) tells member states to set requirements for the implementation of adequate indoor environmental quality standards. Article 13(10)(d) says building automation and control systems must be capable of monitoring indoor environmental quality by 29 May 2026. These paragraphs set no numeric pollutant limits.
That duty attaches to buildings that must have such systems and is applied through national law. Member states had to transpose the Directive by 29 May 2026, and in July 2026 the Commission sent letters of formal notice to all 27 for incomplete transposition, so dates and scope differ by country. More in EPBD indoor environmental quality monitoring: what the May 2026 date means.
Where to measure
A proposed model regulation from REHVA, Eurovent and the Nordic Ventilation Group suggests measuring temperature and relative humidity at 1.1 m above the floor, CO2 and PM2.5 in occupied zones (PM2.5 is not needed where filters control particles and infiltration is insignificant), and outdoor CO2 and PM2.5 as a reference. It is a proposal, not law. When a one-off laboratory test is the better tool is covered in Testing air quality in an office: one-off test or continuous monitoring.
What low-cost sensors can and cannot do
A February 2024 report for the US Department of Energy's Building America program tested four PM2.5 sensor units and four CO2 systems in a chamber. All CO2 systems showed low bias and strong correlation with the reference monitor. All PM2.5 models reported higher concentrations above 20 °C and 60 percent relative humidity, and all underestimated PM2.5 in the presence of interferent dust. The authors say the results should not be taken as representative of the low-cost sensor market.
Peters and Zhen (2024) tested seven consumer monitor models, two units each, in a lab and in a house in Toronto. Their abstract reports acceptable CO2, temperature and humidity for all devices, and only one brand met the PM2.5 accuracy threshold. They did not evaluate TVOC accuracy, calling it complicated to measure. In a Glasgow bedroom, Moreno-Rangel and colleagues (2018) found that the CO2 equivalent a Foobot monitor derives from its TVOC reading gave misleading CO2 levels as an indicator of ventilation.
US EPA's performance targets for air sensors are for non-regulatory use outdoors and at fixed locations, and EPA points to separate ASTM standards for indoor use, such as D8405-21 for PM2.5 sensors. ISO 16000-29:2014 defines test methods for VOC detectors, covering response time, stability and measuring range. Ask a vendor which test method its published results come from. From the evaluations above (general reasoning, not from a standard): trust CO2, temperature and humidity sensors most, treat PM2.5 as a trend that depends on humidity and temperature, and treat TVOC as a flag for follow-up.
Nomestic measures indoor climate room by room (CO2, temperature, humidity and more). It works with or without a building management system, and nothing in the existing HVAC gets replaced. The Airsight Pro Monitor has swappable sensor capsules.
