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Nitrogen dioxide is a byproduct of the burning of hydrocarbons. It is found primarily as a toxic component of vehicle exhaust in any space in which engines are burning gasoline, propane, diesel fuel, propane, natural gas, kerosene, or jet fuel. Areas include enclosed parking structures, ambulance bays, fire halls, warehouses, loading docks, ice arenas, maintenance facilities, and municipal works garages.
Nitrogen dioxide (NO2) is one of a group of highly reactive gasses known as "oxides of nitrogen," or "nitrogen oxides (NOx)." Other nitrogen oxides include nitrous acid and nitric acid. While EPA’s National Ambient Air Quality Standard covers this entire group of NOx, NO2 is the component of greatest interest and the indicator for the larger group of nitrogen oxides. NO2 forms quickly from emissions from cars, trucks, and buses, heavy construction, and off-road equipment.
For vehicle exhaust detection applications, there are a wide array of hazardous chemicals, including particulates, sulfur compounds, carbon monoxide, carbon dioxide, and oxides of nitrogen. As it is not practical to monitor for all of these compounds, a combination of nitrogen dioxide and carbon monoxide sensors will give the best indication of overall air quality.
While the mounting height of CO sensors is often times mandated under building codes, nitrogen dioxide (NO₂) sensors have no standard, regulated mounting height. When deciding where to mount NO₂ sensors, it is dependent on the application and the environment involved.
The density of nitrogen dioxide is greater than that of air, indicating that sensors should be mounted near the floor. However, in instances where large vehicles equipped with top-exiting exhaust stacks, hot exhaust gas will rise to the ceiling, and slowly descend as it cools. For this reason, it is recommended NO₂ sensors be mounted higher up, in order to detect any potentially dangerous levels of NO₂ as quickly as possible.
In applications in which the majority of vehicles include traditional exhaust pipes under the vehicle, we must consider that the hot gas may not reach the ceiling before cooling and descending, and sensors should therefore be mounted at normal breathing level.
In maintenance garages, the height at which employees will be working should also be taken into consideration.
To ensure the safest air quality in a space occupied by diesel vehicles, we recommend that the particular application, the vehicles which will occupy the space, and where personnel will be working should all be taken into consideration before deciding where the NO₂ sensors should be mounted.