Contaminants of Concern (COCs)
Contaminants of concern or COCs are pollutants that could likely harm the building’s occupants. We classify a substance as a COC based on whether it’s known to be harmful. The COC must be common enough so that occupants are likely to breathe the contaminant.
Ventilation successfully controls some COCs but not others. Ventilation is most successful at controlling gaseous contaminants such as water vapor, formaldehyde, acrolein, and NO2. Carbon monoxide from faulty combustion is a gas and cigarette smoke contains toxic gases. However CO and cigarette-smoke concentrations indoors depend more on combustion-appliance performance and the occupant behavior than ventilation airflow.
Carbon Dioxide (CO2) as a Contaminant
CO2 isn’t a very important contaminant for its harmful health effects. The maximum concentrations of CO2 in homes range from 2000-to-5000 parts per million (ppm) and these are short-term and not considered harmful. CO2 is, however, a useful surrogate for occupant-related pollutants in general. Occupants emit CO2 in predictable amounts that relate to other pollutants they emit.
We consider carbon dioxide a contaminant only in concentrations of 3000 ppm or more, which is uncommon in most buildings. Instead we use carbon dioxide as a surrogate or indicator of indoor air quality. We consider occupant activity as a main driving force emitting contaminants. Occupants expel carbon dioxide with breathing so carbon dioxide concentration correlates both to occupant activity and also ventilation performance. Therefore carbon dioxide is the most commonly used controlling variable in Demand-Controlled Ventilation (DCV). The concentration of carbon dioxide controls the DCV airflow with a sensor and a fan control.
Moisture as a Contaminant
Although we group water vapor with contaminants, water vapor isn’t actually a contaminant. Water vapor is just H2O, which composes most of our body weight. However, when water vapor condenses, it provides water for pests such as mold and dust mites. These pests create airborne particles, known as bioaerosols. Therefore, we try to avoid high relative humidity(Rh) inside buildings especially during both the winter heating season and the summer air-conditioning season. Rh is fairly easy to measure with a sensor. The Rh sensor signals the fan control, which either starts and stops the fan or controls its speed.
Ventilation and Particles
Ventilation itself can’t successfully control fine particles (PM2.5). The filters in ventilation systems and in forced air heating and cooling systems can remove fine particles if the filter is rated MERV 13 or more. Filters are especially important when outdoor air contains high concentrations of particles — in cities, windy areas, and on farms for example. When outdoor air contains high concentrations of particles, admits fine particles from outdoors. In that case, increasing the ventilation rate can increase the indoor particle concentration.
Different Sensors for Different Rooms
For bedrooms and living rooms, CO2 and VOC sensors are the most effective. For humid rooms other than toilets, relative humidity sensors are the most effective. CO2 sensors could complement relative humidity sensors in open kitchens that see high occupancies.
For the toilet, the occupancy sensor and the bathroom fan is the best solution to avoid the transmission of odors to the rest of the home. The coupling of light and ventilation switches or VOC sensors could deliver similar good results in toilets.
Further reading
“Indoor Air Quality in 24 California Residences Designed as High-Performance Homes.” (2015).