RELATION OF VENTILATION TO DRAUGHT 245 



HYGIENE. 



RELATION OF VENTILATION TO DRAUGHT. 



Before concluding, I am desirous of considering with you an expression 

 which is in general use, but the frequent cause of wrong views about the 

 change of the air. I mean the word draught. All kinds of complaints are 

 habitually ascribed to it, and the danger of draughts is one of the few 

 hygienic principles which have become thoroughly popular. PerhajDS this 

 was not all profit, because with many people ventilation and draught are 

 Bynonymous; they are afraid of a draught coming from an open window, an 

 open door, and find themselves in collision with ventilation. 



There is certainly and frequently danger in being exposed to a draught — 

 a danger which has, perhaps, been over-estimated, because men have an 

 irresistible desire to fix a certain cause for a certain evil. All collision is 

 avoided if the proper meanings of ventilation and draught are thoroughly 

 understood.;;;;^ 



Ventilation is the necessary change of the air in a closed space, at which 

 the velocity of the air is still taken for a complete stillness, and its motion 

 takes place all around our body. It must not be more than a little above 

 nineteen inches per second. 



Draught is a one-sided cooling of the body, or some part of it, frequently 

 caused by a corresponding motion of cold air, but also in other ways, as by 

 increased one-sided radiation. The danger is, in the first instance, the local 

 perturbation in our heat-economy, which has partly local consequences, but 

 also ai^d chiefly disorders the nerves, acting on the calibre of our blood- 

 vessels, our vaso-motor nerves, which have to regulate the outflow of our 

 heat. . When we are in the open, and the air is in more motion than the air 

 of a draught, we speak of wind, etc., but seldom of draught, because the 

 whole air-current flows equally all round us, just as in a well-ventilated 

 room, only with greater velocity. 



The vaso-motor nerves, regulating the circulation in our skin, are be- 

 yond our control, and we cannot bid them to defend us simply at the place 

 attacked by the draught. They know only how to serve our heat-economy 

 when the outflow of heat from our bodies is equal, or nearly so, on all sides. 

 They misunderstand the local irritation for one spread over the whole surfiice, , 

 and act at once on this error. If one perspires and goes to the window with 

 bared neck or chest, one feels a shiver not only there but all over the body, 

 and the perspiration becomes suppressed accordingly. The blood which at 

 the time filled the blood-vessels of the glowing skin is disj^laced by the con- 

 traction of its channels ; but by the misunderstanding of the vaso-motor 

 nerves it is driven not only from the exposed parts but from the whole sur- 



