In the modern way of thinking, the influence of environmental aerosol on human health represents a partial problem of the manifold interrelations between biological, chemical, and physical environmental factors in terms of their combined and complex effect on the human organism, including the socio-economic environmental conditions. As regards the air as an environmental factor, two fundamental problems are being dealt with in some detail under this particular aspect: the relation between exposure and effect, and the conception of limiting values for the protection of human health. The traditional conception of "one cause--one effect" is inappropriate for dealing with this complex set of problems and has to be replaced by the "many causes--many effects" principle while paying heed to nonspecific as well as indirect effects on the health and well-being of man. The difficulties encountered in clarifying the relation between exposure and effect, and in assessing air pollution from the hygienic point of view are pointed out, and an outline of the present state of knowledge and of the experimental and epidemiological methods of research as applied in this field is given. In conclusion the principles for the determination of air-hygienic standards are expounded.