Can the air go up in flames?
Oxides of nitrogen differ from other combustion-derived air pollutants in that the nitrogen is not coming only from the fuel, but also from the air. Therefore the combustion temperature is extremely important. When fuels burn it is mostly nitric oxide (NO) which is produced, though there may also be some nitrogen dioxide (NO2) formed, and this latter compound is clearly a health risk.
Only a minor part of outdoor nitrogen dioxide originates directly from combustion, most is formed by atmospheric reactions between nitric oxide and ozone. The sources of nitric oxide are in principle the same as those of carbon monoxide, but there is an important difference. Carbon monoxide is most effectively produced during incomplete burning when there is too little air present or the temperature is too low. The production of nitric oxide is best, when there is ample air available and the temperature is high.
The simultaneous control of nitric oxide and carbon monoxide production entails trying to strike a balance between these two opposite phenomena. In large-scale energy production and in internal combustion engines, these processes are controlled well and the emissions have decreased to a fraction of those a few decades ago. Unfortunately the total increases of both energy production and especially the inexorable increase in cars on the road have eaten away the technical achievements.
Most of the nitric oxide emissions originate from energy production, but exposure in the urban environment is most attributable to traffic which is much closer to people than the hundred-meter high chimney stacks of power plants. Long-range transport is also very important. In many areas, some decreasing trends have occurred during the last 15 years, but on the other hand, more polluted urban areas continue to spread out so that more people are exposed to higher concentrations every year. In many European cities, the concentrations are close to the limit values set by the European Union, and in fact often exceed them.
The most important indoor source is the gas stove, but other burning, including tobacco smoking, produces some nitrogen dioxide. Motor engines run indoors may cause very high concentrations, e.g. ice resurfacers and karts.
Nitrogen dioxide is a reactive gas, and therefore only three quarters of the outdoor concentrations can be found indoors, if there are no indoor sources. On the other hand, personal exposure tends to be higher than the outdoor concentration on the street level would give reason to assume.
Nitrogen dioxide together with ozone is the so called deep lung irritant, meaning that these gases penetrate to the lowest parts of the lung including the alveoli. Very high concentrations may cause acute lung oedema known as “silo filler’s disease”. Exposure to high indoor concentrations can evoke symptoms such as cough and runny nose (rhinitis). Nitrogen dioxide may also increase the symptoms of hay fever sufferers and asthmatics to pollen in the air.
What to do?
Urban concentrations are variable, and in cities which control traffic well, the levels of nitrogen dioxide are not a major problem, but in the worst cities the health effects of this gas are quite clear. Making car engines more environmental-friendly will gradually help, but the progress in this respect has been much slower than with sulphur dioxide.
Indoor sources may lead to high concentrations. This should be appreciated by the users of gas heaters and stoves especially if they are used in a relatively small space such as a caravan or trailer. In ice arenas, the final solution is that the resurfacing devices should be equipped with electric motors.
The risks of nitrogen dioxide are quite different in different countries, and depend on climate and traffic emissions. Indoor sources include engines as well as gas stoves and heaters without a flue.
Notes and references
- See the chapter "People don’t die of carbon monoxide poisoning any more – do they?"
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