Where to Mount Nitrogen Dioxide (NO2) Sensors

CETCI recommends that Nitrogen Dioxide (NO2) sensors should be mounted in the breathing zone, four to six feet from the floor. This is primarily because NO2 is a poisonous gas which should be detected in the area where people would be exposed to it, but also because four to six feet from the floor is an appropriate elevation to detect NO2.

Some specs have been written, though, with NO2 sensors mounted near the ceiling. Which is correct? Let’s consider the evidence:

NO2 is heavier than air. NO2 has a molecular weight of 46, compared to air which has a molecular weight of 29.
Graham’s Law describes how gases diffuse and the rate of diffusion. Under normal conditions, gases diffuse rapidly in air. An excellent way to illustrate this is to observe the exhaust from a car starting up on a cold morning. It can be easily seen that the exhaust gases expand upon leaving the tailpipe and rapidly diffuse into the air. In a steady state, a gas will diffuse throughout a space until it reaches equilibrium.
Hot exhaust gas will tend to rise initially because of its lower density, but the kinetic energy of the exhaust gas molecules, combined with any air movement in the space will cause the gases to disperse throughout the space.
It is true that diesel exhaust from truck exhaust stacks is relatively high off the ground. However, many vehicles with diesel engines have their exhaust pipes near the ground, including most fire trucks.
All internal combustion engines produce Oxides of Nitrogen, including NO2, as a component of vehicle exhaust. So exhaust from cars and light trucks must be taken into account when monitoring for the presence of NO2. These vehicles, of course, have their exhaust pipes near to the ground.
NO2 is a very toxic substance and can cause health problems at very low concentrations. Worksafe BC, the British Columbia workplace health and safety authority, has set a 1 ppm ceiling for NO2 exposure. NIOSH has set the NO2 short term exposure limit at 1 ppm. So worker safety is a vitally important reason to monitor for the presence of NO2, in the area where workers are most likely to be exposed to it.

Considering all the evidence, mounting NO2 sensors four to six feet from the floor, in the breathing zone, is appropriate and backed up by the facts.