The monitoring units detect the fumes emitted by the cement plant
It depends on where they are located and what other man-made sources are present in the area.
It is important to remember that most of the emissions of NOx, PM10 and PM2.5 are not caused by a single plant, but mainly from third-party sources like vehicle traffic, civil heating and agriculture.
Like all emissions, a plant's emissions are dispersed due to orography, winds and atmospheric conditions that can result in concentrations on the ground – even at the points of maximum fallout – that are as much as 200,000 times lower than the values measured at the stack.
Mathematical modelling is now able to represent the immission load of pollutants from a specific source in an extremely precise manner and to compare it with what was found by environmental monitoring networks installed and managed by bodies like ARPA or by municipalities.
The monitoring devices, even if positioned at the points of maximum fallout, will also detect all other fallout at human height from other sources of emissions in the area.
In the worst cases the weight of the pollutants produced by the cement plant and detected by the monitoring devices will account for just a few percentage points of the total.