smoke, visible gaseous product of incomplete combustion. Smoke varies with its source, but it usually comprises hot gas and suspended particles of carbon and tarry substances, or soot. To reduce the amount of smoke entering the atmosphere, air pollution laws generally require that power plants, factories, and other large combustion facilities burn anthracite (hard) coal, natural gas, or low-sulfur fuel oil rather than bituminous (soft) coal or high-sulfur fuel oil, and that smokestacks be equipped with scrubbers or other devices. Proper firing techniques and equipment can eliminate or greatly reduce the smoke produced by any fuel. Wood gives little smoke if burned when dry and if the fire is given a good supply of air. Where it is necessary to use soft coal because of its lower cost or because other fuel is not available, the grate and flue must be built to insure maximum combustion, the coal supply must be carefully regulated, and adequate air must be supplied. There are various ways of reducing the amount of smoke escaping into the air. Some methods utilize electricity or sound waves for precipitation of the suspended particles, others employ chemicals; the method using an electric current at high potential is perhaps best known. Smoke precipitates may yield valuable byproducts; for example, fly ash can be used as a construction material. Among the evils of smoke are interference with sunlight, causing the most healthful rays of the sun to be filtered out and necessitating the use of artificial light; disfigurement of buildings, leaving deposits that are costly to remove and causing corrosion of stone and metalwork; destruction of plant life by shutting out sunlight and by clogging the stomata of leaves with oily deposits; and injury to the respiratory systems of humans and livestock. Tobacco smoke, in particular, is known to be related to cancer of the lungs and other organs (see smoking). In addition to such damages, smoke also represents a waste of energy, as imperfect combustion dissipates potential heat into the atmosphere. Smoke particles and other air pollutants are often trapped in the atmosphere by a combination of environmental circumstances (see temperature inversion), forming smog. Paris early passed stringent laws in an effort to preserve architectural and sculptural monuments, and most U.S. cities had smoke-nuisance laws before air pollution regulations were put into effect. Smoke-nuisance laws are difficult to enforce and often are not applicable to existing residential heating units, although these are often important contributors to pollution. In order to comply with federal air pollution standards many cities have now adopted building codes that require minimally polluting heating units in new buildings and that forbid the use of incinerators.