» Children are particularly vulnerable to poor air quality «
Airing with windows in the morning, afternoon and before bedtime will help create good indoor air quality in the house. Airings can also be controlled via sensors, which is the most efficient way of providing good indoor air quality with windows and natural ventilation.
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; breathing generates CO2, so the indoor CO2 concentration will always be at least 400 ppm and usually higher. An indoor CO2 level of 1 150 ppm provides adequate air quality, 1 400 ppm will ensure good indoor air quality in most situations, and 1 600 ppm indicates poor air quality (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).
As humidity is considered to be the main pollutant in homes, it can be relevant to keep the indoor humidity level under observation. For some rooms, this can be done via the relative humidity but, in more advanced systems, the difference in humidity content between the indoor and outdoor air can be evaluated and used as an indicator.
Measuring relative humidity has been done for many years and is now a market standard. The indoor levels in cold climates are generally high during summer and lower during winter. With the same ventilation rate during summer and winter, the indoor relative humidity will be very different from summer to winter. In other words, a fixed relative humidity as indicator for Indoor Air Quality has, some limitations and is most useful in wetrooms, where the objective is to avoid very high levels of humidity. Relative humidity is very relevant as indicator in bathrooms, and in kitchens.
In terms of absolute humidity, however, the difference between indoor and outdoor humidity content may be the best indicator, even though this will require indoor and outdoor sensors. In this case, a difference of 3.5 g of water vapour per m³ of air is a reasonable level, and may be used all year to check if the humidity production in the home is balanced correctly with the ventilation rate. Measuring the difference in absolute humidity is not a market standard, so there are few products on the market.
VOC as indicator for air quality
Volatile Organic Compounds (VOCs) are substances that evaporate easily and are a mixture of many different chemicals such as benzene, formaldehyde and trichloroethylene (TCE). The effect on humans ranges from experiencing upleasant smells to severe health effects, e.g. as a cause of cancer.
There are two kinds of VOC sensors on the market: one that measures the actual VOCs in the air, registering odours, cooking and smoking fumes, and solvents; and one that correlates VOC levels with CO2 levels coming from human activity which also generates VOCs. This fact, combined with the ability to detect smells, could make VOC sensors an alternative indicator for air quality to CO2 as the VOC sensor is often cheaper in price.
It is generally difficult to quantify the limit levels of VOCs, which is more commonly often used in scientific circles; whereas VOC sensors correlating with CO2 levels could be a good alternative to existing CO2 sensors that evaluate human occupancy in buildings.