2.1.2 Indoor air quality indicators
As described earlier, indoor air contains many pollutants. For many years, discussion has continued as to which indicator for indoor air quality is the most suitable. Carbon dioxide (CO2) is probably the most commonly used indicator, measuring the CO2 produced by human breathing and emitted by appliances such as gas cookers and boilers (CIBSE, 2011). Other indicators are humidity and volatile organic compounds (VOCs), both of which are possible indoor air quality indicators.
CO2 as indicator of air quality
Carbon Dioxide is often used when talking global warming as it is one of the main greenhouse gasses causing global warming. But, CO2 is a good indicator of the indoor air quality in houses, where the occupants and their activities are the main source of pollution, as CO2 is emitted by all humans while breathing and not by many other sources. However, CO2 is rarely a health issue in itself. It is nevertheless a very good indicator of human presence and the level of ventilation. Outdoor air contains approximately 400 ppm of CO2; breathing generates CO2, so the indoor CO2 concentration will always be at least 400 ppm and usually higher, especially in bedrooms.
According to European standard, EN 16798-1:2019, there are given 4 categories of the expected indoor air quality, ranging from category I (high expectation) to category IV (low expectation). This standard is not necessarily legislation unless referred to in national legislation.
For CO2 these levels are category I (950 ppm), category II (1200 ppm), category III (1750 ppm) and category IV (above 1750 ppm). Though for bedrooms these levels are lowered in the standard. An indoor CO2 level of max 1200 ppm therefore provides a medium IAQ expectation which is generally acceptable and above 1750 ppm will give a low expectation of the IAQ (CEN, 2019; Active house Alliance, 2020). CO2 is most relevant as an indicator in rooms where the need for ventilation is linked to the presence of people, e.g. in bedrooms, children’s rooms, living rooms, dining rooms, classrooms and offices.
Humidity as indicator of air quality
The relative humidity indoors will vary on a yearly basis in correspondence with the humidity level outdoors. A high level of humidity in indoor air can increase the presence of house dust mites. So in climates with cold winters, the relative humidity inside should be kept below 45% during winter (Richardson et al, 2005). Generally speaking, high relative humidity levels should be avoided in order to limit the risk of mould growth, with negative health conditions such as asthma and allergies as a consequence (Liddament, 1996).