Minor particles find their way down to the sensitive small alveoli of the lung where the exchange of oxygen and carbon dioxide takes place. Particles normally have an organic layer, i.e. toxic substances are absorbed on the surface of the particles. Gases are inhaled and, to a large extent also exhaled, but there is potential for reactions to occur. For instance, nitrogen dioxide can react with water in the mucous membranes and form nitric acid which can result in different lung reactions. Many air pollutants, especially particles and metals, precipitate on soil and can contaminate vegetables and grass and thereby reach humans via the food chain.
Water pollutants originate in many cases from sewage water from urban areas and industrial plants.
The contaminants are either released into the sewage water or are subsequently formed in the aqueous recipient. An indirect consequence is consumption of oxygen, when the organic material is metabolized by microorganisms, with severe consequences on life in general. Many substances precipitate on the bottom where organisms are contaminated and eaten by other organisms. Finally, the contaminants accumulate in the upper part of the food chain and are eaten by man. The recipient can be rivers, lakes, oceans or ground water.