Spread in air, dust represents solids of various kinds, forms, structure and density and can be subdivided in accordance with particle size. In moving air, dust does not obey the laws of gravity but is precipitated slowly when the air comes to rest. Soot is flocculated coal dust which arises from incomplete combustion and has a corrosive effect, particle size 2 - 6 micron. Aerosols are solid or liquid substances very finely distributed in gas, particle size 0.01 - 0.1 micron. Smog, arising from the accumulation of very fine particles in air, may lead ultimately to bad visibility. Fog arises from finely distributed droplets of water in air, particle size 1 - 50 micron.
Diagram 1.7.1.1
These are only the most significant types of solid particles in air. Dust is composed from organic constituents such as seeds, pollen, textile fibres or even flour, as well as from inorganic matter such as sand, soot, ash, chalk and stone dusts. Dust is created in a natural manner through weathering and decay, putrefaction and fires, or through everyday human activities. Particularly also, through highway and railway traffic or from industrial activities in cement works or the chemical industry as well as through mining, to cite only a few examples. The concentration of dust in air depends greatly on weather conditions such as wind and rain as well as the time of day and the season and, of course, differs from region to region.
Table 1.7.1.1 - Average Dust Content in Air| Location | Medium Concentration mg/m3 | Average Particle Size µm | Largest Particle Size µm |
|---|
| |
| Rural Area | | | |
| During Rain | 0,05 | 0,8 | 4 |
| Dry Weather | 0,15 | 2 | 25 |
| |
| Large City Area | | | |
| Residential | 0,40 | 7 | 60 |
| Industrial | 0,75 | | |
| |
| Industrial Area | 3 | 60 | 1000 |
| Workshops | 1 - 10 | - | - |
| Fettling Shop, Foundry | 50 - 100 | - | - |
| Cement Factory | 100 - 200 | - | - |
| Combustion Flue Gases | 1000 - 1500 | - | - |
The upper acceptable limit of dust deposition has been laid down: in a Spa area 2 - 10 g/m3 and more in general 10 - 15 g/m2 and more in industrial areas 20 - 30 g/m2 and more When investigating the vertical distribution of dust, it was found in areas of high concentration that the first dust layer was about 3 - 4 m above the earth’s surface in the form of traffic dust, with a second layer above which contained mainly heating dust. Apart from somewhat impairing breathing, the dust normally contained in air does not damage health because the mucus membranes in the respiratory ducts have a filtering effect.
Table 1.7.1.2 - Grain Sizes of Dust in an Industrial Area| Size Range | Medium Size | Number per m3 | Percentage weight/ volume |
|---|
| µm | µm | in 1000 | % |
|---|
| 10 - 30 | 20 | 50 | 28 |
| 5 - 10 | 7,5 | 1750 | 52 |
| 3 - 5 | 4 | 2500 | 11 |
| 1 - 3 | 2 | 10700 | 6 |
| 0,5 - 1 | 0,75 | 67000 | 2 |
| 0 - 0,5 | 0,25 | 910000 | 1 |
However, dust from production areas can be damaging or dangerous (such as silicosis) for living organisms and calls for targeted, effective reduction of the dust load in order to maintain viable living conditions. Dangerous dust impairs respiration, causes allergic reactions, impedes sunlight and, in conjunction with moisture and in the course of time, damages valuable buildings by decomposition.
