In a previous post, I discussed my plans for buying an outdoor quality sensor. But, I didn’t explain what air quality actually entails. It isn’t just one thing. There are a lot of factors involved. The US Environmental Protection Agency sets an air quality index for five major air pollutants:
- ground-level ozone – created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). Ozone is most likely to reach unhealthy levels on hot sunny days in urban environments, but can still reach high levels during colder months. Ozone can also be transported long distances by wind.
- particle pollution – This most commonly includes PM2.5 and PM10. The 2.5 and the 10 indicates the size of of particulates and the measurement is of the concentration in the air
- carbon monoxide – The most common source of CO outdoors would be cars, trucks and other vehicles or machinery that burn fossil fuels. Indoors, gas appliances, furnaces, and chmineys.
- sulfur dioxide – The most common source of SO2 is burning of fossil fuels by power plants and other industrial facilities, as well as some heavy equipment.
- nitrogen dioxide – NO2 forms when fossil fuels such as coal, oil, natural gas or diesel are burned at high temperatures. Again, cars are the most common one here.
These are all a concern indoors as well to some degree, but there are additional items that are a focus indoors:
- carbon dioxide – CO2 is produced both naturally and through human activities, such as burning gasoline, coal, oil, and wood. People exhale CO2 which contributes to CO2 levels in the air. Why is this important indoors? It is often measured to quickly but indirectly assess approximately how much outdoor air is entering a room in relation to the number of occupants. During the pandemic, as a measurement of air circulation, using this as a way to determine how well ventilated a space is. When I was in one older building with a group of people, the bulding had a protocol that, if the levels grew too high, they would open windows to get it down.
- formaldehyde – HCHO is found in some building materials, including composite wood, insulation, glues, paints and finishes, preservatives, pesticides, cigarette smoke, etc…
- volatile organic compounds – emitted as gases from certain solids or liquids and are emitted by thousands of products such as paint, solvents, aerosol sprays, cleansers and disinfectants, air fresheners, automotive products, hobby supplies, dry-cleaned clothing, pesticides, copiers, etc.
Scary stuff. Most indoor air quality sensors contain a TVOC sensor, a PM2.5 sensor, and a CO2 or eCO2 sensor.
- TVOC – because there are so many VOCs, it’s impossible to monitor them all. TVOC is a measurement used to measure the overall amount of VOCs in a space. It is not uniformly defined.
- PM2.5 – As mentioned before, this is the size of the particulates. Anything PM10 or less is inhalable. Fine particulates are PM2.5 or less. Therefore, the PM10 reading contains the PM2.5 particulates as well. Combustion of gasoline, oil, diesel fuel or wood produces most of the PM2.5 pollution in the air, and PM10 also includes dust from construction sites, landfills and agriculture, wildfires and brush/waste burning, industrial sources, wind-blown dust from open lands, pollen and fragments of bacteria.
- CO2 vs eCO2 – Estimated CO2 is a derived number based on the TVOC reading of a sensor. . If there are substantial concentrations of other VOCs present, the eCO2 reading would be higher than the actual CO2 level.
If you remember from that previous post, my on-order outdoor sensor has dual PMS5003T sensors, which measure PM2.5 and PM10, an SGP41 for VOC and NOx measurements, and I could opt to add a NDIR CO2 sensor. In fact, after I ordered, they switched from dual particulate sensors to 1 Particulate and one CO2 sensor for their outdoor kit. While elevated CO2 levels are usually used to derive indoor air circulation and quality, they can also be used outdoors to indicate other harmful gases that are often emitted with CO2 like SO2, NO and NO2 as they are often emitted with them. Similarly, VOC is more commonly measured on indoor sensors, but can also appear outdoors near chemical factories, gas stations, natural gas leakages, and burning of garbage.
Air Quality is derived from the concentration of these items in the air and usually displayed as a number on the Air Quality Index. Different countries have different scales and formulas for calculating this. The US EPA’s scale is 0-500 and uses color coded ‘traffic light’ system to indicate good versus bad air.
- 0-50 is Good(Green)
- 51-100 is Moderate (Yellow)
- 101-150 is Unhealthy for Sensitive Groups (Orange)
- 151-200 is Unhealthy (Red)
- 201-300 is Very Unhealthy (Purple)
- Above 300 is Hazardous (Maroon)