The EPA (Environmental Protection Agency [ USA ]) and the EEA (the European Environmental Agency) are both in clear agreement, NOx is bad for people’s health.
The European Environment Agency (EEA) estimated that in 2015 over 72,000 premature deaths in the 28 EU member countries were a consequence of NO2 levels in the atmosphere.
But NOx does not only mean NO2. NOx is a shorthand term referring to the oxides of nitrogen. Some of these are formed naturally, but the majority of the oxides that plague our atmosphere in our towns and cities are products of our society, and most notably come from combustion. At the high temperatures found in combustion processes, the nitrogen molecules in the combustion air (N2) are broken into ions, that then react with oxygen molecules (O2) to form oxides of Nitrogen (NO, N2O, NO2…). Of these oxides by far the most common that is formed during combustion is NO (nitric oxide).
As it goes, NO is quite dangerous as it is. Exposure to nitric oxide gas in low concentrations produces an irritating effect on the mucous membranes of the eyes, nose, throat and lungs, which can include choking, coughing, headache, nausea and fatigue.
But it does not stop there, The NO molecules react together with UV light and volatile organic compounds (VOCs) in the atmosphere. This reaction reduces the size of the VOC molecule and converts the NO to NO2 (Nitrogen Dioxide). NO2 is a particularly nasty air pollutant and causes significant respiratory problems even at the 40 – 100 µg/m3.
The cycle continues as NO2 in the air reacts in the presence of oxygen and UV light to create ozone (O3). The same reaction also converts the NO2 molecule back to NO. The molecule of NO is now free again to find more VOC compounds to repeat the cycle. A typical VOC compound can recycle a molecule of NO multiple times (on average 5) before it ceases to be photo reactive.
The gases that are produced during this cycle; ozone, NO and NO2 all cause respiratory issues, and at elevated levels can rapidly cause direct harm to tissues. NO is relatively harmless below 25ppm, but NO2 concentrations higher than 4ppm will already anaesthetise the nose.
Although Ozone is desired in the stratosphere (the higher levels of our planet’s atmosphere) to help protect us from the effect of UV light, tropospheric ozone (that is ozone in the air that is closest to the earth’s surface) has been and continues to be a significant air pollution problem and is a primary constituent of smog. There is a great deal of evidence to show that ground level ozone can harm lung function and irritate the respiratory system. Exposure to ozone NO and NO2 is linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems.