To the consumptive himself these measures are not without a vital significance. For his chances of recovery may be in no small degree diminished if he be more or less constantly liable to a fresh infection from material which he has once got rid of, and which should have been destroyed.
The great volumes of fresh, moving air which we encounter out-of-doors in properly cleansed streets usually so greatly dilutes the dust, of whatever kind, that little apprehension need be felt from its presence. When, however, in crowded cities, the streets are, as is nearly always the case, save for a few favored localities, filthy, and but fitfully cared for; when choking dust clouds must be encountered by the citizen during the haphazard and slatternly essays at cleaning made by untrained and irresponsible sweepers; we cannot ignore a danger from street dust which may well incite grave apprehension. The citizen can, if he must, run from the presence of cloud-enwrapped machines furiously whirled along half-sprinkled pavements; he may avoid a block on which the hand-sweepers, in the absence or in disregard of rules, ply their nefarious brooms over unwet surfaces, because too indolent or indifferent to sprinkle them--these things he can do if he be not willing or ready to apply the citizen's remedy for municipal misrule.
But it is in rooms either of dwelling or assembling places that the ill effects of infectious dust are most potent, because the air is here not so constantly renewed as it is out-of-doors, and is liable to be breathed over and over again. Dust which gets into houses does not readily leave them, unless special and intelligent means be directed to its removal. We do not usually realize that, though the air itself in inhabited rooms is constantly changing more or less rapidly by diffusion, by draughts, or by purposed ventilation, fine dust particles are not removed under the same influences in proportionate degree. They cling more or less tenaciously to all surfaces on which they have settled, and especially to fabrics, so that currents of abundant force and sufficient distribution to change the air may and usually do leave the lodged dust particles almost entirely undisturbed.