Air of a high relative humidity is objectionable from a hygienic standpoint because it interferes with perspiration and evaporation. Workers constantly exposed to atmosphere that is too damp will have a tendency to incoördinations involving the bodily secretions and respiratory disorders. The ill effects of such an environment are rather marked and if it is necessary to maintain a high relative humidity because of the processes carried on there must be special provision to guard the health of the workers. This may be accomplished by short hours and frequent periods of rest and relief from the high humidity. Normal air pressure is fifteen pounds to the square inch, but there are many industries in which the work must be done under a greater or a less pressure. Mountain climbers are subjected to decreased air pressure while caisson workers and divers must work under greatly increased air pressure. Decreased air pressure is especially detrimental to those suffering from cardiac disorders, and may in some cases result in death. The effects of increased air pressure have been elaborated upon in another chapter and the student is referred to that section. The location of the work is important. Subsurface work is more detrimental to health than surface work unless special precautions have been taken to make the place of work conducive to the expression of life. Subsurface workers such as miners are subjected to great heat, lack of proper light and ventilation, accumulated gases, dampness and poisons. There is also greater danger from accidents and explosions. Industrial Dusts In a trade in which much dust is produced there should be proper mechanical ventilation. The vacuum and plenum system should be employed. There is no factor in the industrial life more detrimental to the workman than that forcing him to breathe vitiated air laden with dust and other impurities produced by the processes employed in the trades. The extremely dusty trades should be separated from the less dusty ones and special provision made to carry away the dust. It is often possible to substitute machinery for handwork in these trades. When the laborers are forced to work in very thick dust they may be protected in a measure by the use of respirators worn over the mouth and nose. These will catch some of the dust and act as filters. It is often possible to place hoods directly above the machines in these trades and by means of a vacuum fan the dust is carried out through tubes. The effects of dust upon the worker depend upon many factors. These might, however, be classified as conditioning factors in the individual and the character of the dust inhaled.