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Important among the various sources of emission are domestic heating and motor vehicles. Incomplete combustion causes most emissiun. Macro-climatic and micru-climatic conditions are highly significant in the spread and density uf air pollutiun. Although unly unpleasant smell or soiling air pollutiun have caused offen ce, we must recognize and examine the main harmful gaseous and powdery ingredients, particularly thuse with lasting harmful effect. Thus, of interest amung powdery emissions are only the suspended particles 5 p), because only they can penetrate the lungs. Sedimentation methods are therefore nut suited to infurm us un dust aero- soles injurious to health. The must important emissions are cancerogenic substances such as 3,4-benzpyren. Under special conditiuns, animal experi- ments shuw that these substances are able to cause bronchial tumours. The increase in lung cancer appears tu be assuciated with high air pullution in cities; the must frequently and increasingly proved gaseous emissions are S02, CO and N0 . 2 Mure impurtant than the individual components is the examinatiun of the combinatiun of various gaseous and puwdery air pollutiuns. The catastrophes in London, Donora and in the Maas valley, and special conditions in Los Angeles, are significant examples uf the importance uf air hygiene. Towards the maintenance of clean air, filters can be mounted in chimneys, although filter efficiency drops appreciably with a grain size uf
