Indoor air quality (IAQ) describes the state of the air we breathe inside a building: its pollutant content, how often it is renewed and its thermal comfort. In an office, that air directly shapes the health, comfort and focus of the people who spend a large part of their day there.
The short answer: it is measured with CO2, particulate matter, temperature and humidity sensors, and the main lever to improve it is ventilation, that is, replacing indoor air with outdoor air. In Spain the reference framework is the RITE (Regulation of Thermal Installations in Buildings), which sets air quality categories and minimum airflow rates for each type of space.
This article explains which pollutants to watch, why IAQ matters for productivity and absenteeism, how it is measured in practice and what Spanish regulation requires.
What indoor air quality is
Indoor air quality is the result of a balance between the pollution sources inside a building and the ability of the ventilation system to dilute and remove them. We spend most of our time indoors, so the air in offices, classrooms and homes tends to influence health more than street air does. Poor IAQ shows up as stale air, fatigue, headaches, dry eyes or trouble concentrating, a set of symptoms historically linked to sick building syndrome.
Common indoor pollutants
The most frequent pollutants in a workplace are:
- Carbon dioxide (CO2): exhaled by people. It is not toxic at office concentrations, but it is an excellent indicator of ventilation levels and of the build-up of human bioeffluents.
- Volatile organic compounds (VOCs): released by furniture, paints, varnishes, adhesives and cleaning products.
- Particulate matter (PM2.5 and PM10): airborne particles from outdoor air, traffic or certain indoor activities.
- Relative humidity: if too high it encourages mould and dust mites; if too low it dries out the airways.
- Carbon monoxide (CO), radon in some geographic areas, and allergens such as pollen or spores.
The sources are varied: human occupancy itself, materials and furniture, cleaning products, office equipment, the air coming in from outside and a poorly maintained HVAC system. Good HVAC maintenance matters as much as the design of the installation itself.
Why indoor air quality matters
Beyond comfort, IAQ has measurable effects on health and performance. Poorly renewed air is associated with fatigue, headaches and respiratory discomfort that can translate into lower wellbeing and more absences. Conversely, clean, well-ventilated air supports concentration and cognitive performance.
The Harvard COGfx study, published in Environmental Health Perspectives in 2016, measured the cognitive performance of office workers under different environmental conditions. Against a day in a conventional building, cognitive scores were 61% higher on the green building day and 101% higher on the days with enhanced ventilation and lower VOC and CO2 levels. These figures are best read as a signal of the direction of the effect, not as an exact promise for every office.
For a company, this connects people's wellbeing with productivity and with metrics such as absenteeism, and also with building management: more ventilation uses energy, so IAQ is part of the conversation about energy efficiency and about the energy impact and cost for companies.
How indoor air quality is measured
Measuring IAQ means installing sensors that continuously monitor the key parameters and make it possible to act on ventilation. The most common are:
- CO2 sensors: the most widespread proxy for ventilation level. Their use became popular after the pandemic as a simple way to estimate air renewal in occupied spaces.
- Particulate matter sensors (PM2.5 and PM10): to watch particles entering from outside or generated indoors.
- Temperature and relative humidity sensors: to ensure thermal comfort and avoid conditions that favour mould.
Outdoor CO2 is around 400 ppm, and 1000 ppm is usually cited as the maximum recommended level indoors: above it, ventilation is likely insufficient for the number of people present. Real-time sensors also allow automatic control of ventilation based on occupancy (known as demand-controlled ventilation), so the space is ventilated more only when needed, saving energy the rest of the time.
The framework in Spain: the RITE
The RITE, the Regulation of Thermal Installations in Buildings, governs ventilation and air conditioning of buildings in Spain. To classify indoor air quality it defines four categories, called IDA, based on the CO2 concentration measured in ppm above the outdoor air concentration.
| IDA category | Air quality | CO2 above outdoor (ppm) | Typical use |
|---|---|---|---|
| IDA 1 | Optimal air quality | 350 ppm | Hospitals, clinics, laboratories, nurseries |
| IDA 2 | Good air quality | 500 ppm | Offices, classrooms, museums, reading rooms |
| IDA 3 | Medium air quality | 800 ppm | Shops, restaurants, computer rooms |
| IDA 4 | Low air quality | 1200 ppm | Other, less demanding uses |
Offices must reach the IDA 2 category (good air quality). To guarantee it, the RITE also sets minimum outdoor airflow rates per person. At low metabolic activity, IDA 1 requires 20 dm³/s (l/s) per person and IDA 2 requires 12.5 dm³/s per person. In practice, for an office the minimum renewal rate is 12.5 l/s per person.
Complying with the RITE is the basis for a building to offer healthy air and, done well, to do so without driving up energy use.
How to improve indoor air quality
Measures to improve IAQ combine ventilation, filtration, source control and monitoring:
- Ventilate properly: guarantee the outdoor airflow rates the RITE requires and prioritise ventilation when CO2 rises. Ventilation is the main lever.
- Filter the air: use suitable filters in the air conditioning system to capture particles from outdoor air, and keep them clean.
- Control the sources: choose furniture, paints and cleaning products with low VOC emissions, and avoid loading the air unnecessarily.
- Manage humidity: keep relative humidity within a comfort band to prevent mould and respiratory discomfort.
- Monitor continuously: install sensors and, where possible, link ventilation to actual occupancy through automatic control.
- Maintain the HVAC system: regular maintenance keeps the installation itself from becoming a source of pollution.
Many of these measures have a second, environmental reading. Ventilation and air conditioning use electricity, so optimising ventilation cuts both consumption and the electricity footprint of the building, with a direct impact on its carbon footprint and its greenhouse gas emissions. Powering those systems with renewable energy and tracking progress with sustainability indicators helps ensure that improving the air does not worsen the environmental impact. Building certifications such as WELL or LEED integrate exactly these health and sustainability criteria.
Frequently asked questions
What CO2 level is acceptable in an office?
Outdoor CO2 is around 400 ppm. Indoors, 1000 ppm is usually taken as the maximum recommended level; above that value it is worth reinforcing ventilation. The RITE classifies offices as IDA 2, which means keeping CO2 about 500 ppm above the outdoor air.
Is CO2 dangerous at those concentrations?
No. At typical office levels CO2 is not toxic. Its value lies in being a good indicator: when it rises, it signals that ventilation is not renewing the air at the pace the people present require and that bioeffluents are building up.
Which indoor pollutants should be watched besides CO2?
Above all the volatile organic compounds (VOCs) from furniture and cleaning products, particulate matter (PM2.5 and PM10), and relative humidity. In some areas radon too, and always CO where there is combustion.
What does the RITE require regarding ventilation?
The RITE classifies air quality into four IDA categories based on CO2 and sets minimum outdoor airflow rates per person. Offices must reach IDA 2, with a renewal rate of 12.5 l/s per person at low metabolic activity.
Improving indoor air quality means ventilating more, and ventilating uses energy that ends up as emissions. Manglai does not measure air quality, but it does quantify how much air conditioning and ventilation weigh in a building's consumption and emissions, through its carbon footprint platform.
