Many different instruments may be used to measure air pollution, one of them being the diffusion tube. This is a simple device that consists of a small plastic tube with metal gauze that is coated with a substance that can absorb nitrogen dioxide to measure the levels of the gas.
While many air pollution devices are known as active devices, the tubes are known as passive diffusion tubes since they do not require the use of pumps. Various types of devices are used to detect different pollutants from the air. In many parts of the United Kingdom, for example, there is a station in each major city or town that has specialized instruments for this purpose. This is necessary because the levels are considered to be at or above what has been deemed as healthy levels of the pollutants. They can be quite expensive to buy and maintain, even though they are fairly small.