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NO2 is a highly reactive gaseous compound with a reddish-brown color and pungent odor. It is the main chemical form with adverse health effects within the nitrogen oxides. It is generally produced due to the oxidation of N2, the main component of air, at high temperatures. Its production is related to combustion processes, traffic, transportation and high temperature industrial processes and power generation. In addition to being a common outdoor pollutant, it also occurs in indoor spaces with gas stoves, combustion heating and as a constituent of tobacco smoke.
Adverse health effects range from airway inflammation to more significant effects such as lung infections and respiratory failure. In addition, they can act as precursors of ozone and other photochemical pollutants such as volatile organic compounds (VOCs), in turn causing adverse health effects, as well as damage to the respiratory tract and other systems such as the cardiovascular and immune systems, among others.
The limits established in Spain for short-term exposures (VLA-EC), referring to averages of 15-minute periods, are 1 ppm. The limits for daily exposure (VLA-ED) for a standard 8-hour working day are 0.5 ppm. On the other hand, the hourly limit value for the protection of human health is more restrictive and is set at 105 ppb, while the annual limit value is 21 ppb. The WHO, on the other hand, sets a limit of 5 ppb for average annual exposure, and 13 ppb for average daily exposure. On the other hand, the EPA sets an annual level of 53 ppb for health protection. These are all outdoor exposure limits, as there are no normative reference values for indoor spaces, beyond WHO recommendations or private standards such as WELL certification.
The ranges of values used in the MICA device as health effect indicators are based on the limits in Spain for the protection of human health.
The NO2 concentration can be measured either in µg/m³, ppm or ppb. MICA displays these data in ppb, but conversion to other units of measurement can be easily performed, as long as the indoor environment conditions are considered. Considering a typical room at a temperature of about 20°C, the conversion to be used is as follows:
- 1 ppb ~ 1,913 µg/m³ or 1 µg/m³ ~ 0.523 ppb
NO2 is mainly produced in the outdoor environment, due to combustion processes that take place in connection with transport vehicles and, above all, in cars with diesel engines. More than 75% of NO2 in ambient air is usually due to traffic. In addition, external sources also include high-temperature industrial and power generation facilities, where fossil fuel combustion processes such as coal, gas and oil take place.
Regarding indoor sources, NO2 concentrations can be greatly increased if fossil fuel stoves and heaters, such as gas stoves or kerosene heaters, are used. Higher concentrations are also obtained in homes with gas stoves. Tobacco smoke is another major source of NO2 indoors.
Low NO2 levels are beneficial to health. First, low concentrations are an indication that the indoor environment is safe and healthy. Exposure to NO2 can cause respiratory problems, especially in at-risk individuals such as infants, children and the elderly, as well as people with asthma and respiratory conditions. In addition, the risk of spreading viruses is also reduced in spaces clean of this substance. In terms of other risks to human and environmental safety, NO2 control can prevent accidents such as fires or explosions.
The risks of NO2 concentrations outside the recommended ranges are diverse in nature. It is classified from harmful to toxic by inhalation depending on the concentration of NO2 in the inhaled air. It is also included in the ISTAS blacklist as an endocrine disruptor. In addition, it is considered one of the six pollutants listed as most critical in terms of IAQ effects according to NAAQS.
Harmful effects start with immediate irritation of the respiratory tract. In the short term, it can lead to airway inflammation, coughing, shortness of breath and can also aggravate asthma and other respiratory conditions. In the longer term, if exposure is prolonged, it can lead to chronic problems, develop asthma and increase susceptibility to infections, as well as lead to lung infections and respiratory failure. It can also lead to organ disorders such as liver, spleen, or problems in other systems such as the circulatory and immune systems.
Finally, the risk of fire and explosions posed by high levels of NO2 is also considered. In addition, due to its high reactivity, it reacts with chemicals in the air to form PM, O3 and TVOCs, which in turn have other harmful effects on health and the environment.
