Unpleasant odours can trigger stress reactions in people, cause discomfort and distress, and may provoke a variety of psychological stress responses.
Unpleasant odours may be produced as a result of industrial processes, fuel combustion, processing of animal products, improper disposal of liquid and solid wastes or lack of personal hygiene. In streets carrying heavy traffic, and under certain meteorological conditions, bad odours may be given off by motor vehicles. Such malodorous substances, though not clearly defined, are principally products of incomplete combustion and various components of fuels. The concentration of such substances may be very low but they may greatly distress pedestrians and the occupants of buildings along the roadside. Despite the reassurance of toxicologists, the public tends to fear such odours not only because of the discomfort they cause but also because they are believed to be a threat to health.
There is no single statute aimed specifically at the control of odour emissions in the way that limits are set for the discharge of substances with known effects on people, vegetation, the fabric of buildings and other materials; so there are no targets for industry to meet. This unsatisfactory situation over smells arises mainly because their environmental nuisance cannot be described in unequivocal, measurable terms. Moreover, most smelly substances are odorous at concentrations of 10 parts per million, some at less than 0.001 parts per million, or at a fraction of the quantities adopted to safeguard individuals against the toxic properties of compounds.
The Paris metro has investigated the implementation of industrial perfumes to make passengers more comfortable and thereby improve the image of the metro. The chief source of unpleasant odours on the metro are the rotting bacteria trapped in the concrete when the system was built.