LIGHT THERAPY MAYER 403 



erythema is caused by rays from 302 to 297 millimicrons. Four per 

 cent of this maximum is at 313 millimicrons ; 45 per cent is around 

 253 millimicrons. Erythema is not caused by even one hour's 

 exposure of the skin to ultra-violet rays longer than 330 millimicrons 

 (Hill). Rays shorter than 250 millimicrons produce a very faint 

 erythema. Dosage in therapy with unfiltered lamps must therefore 

 be regulated by the intensity of wave-lengths from 313 to 250 milli- 

 microns present. Erythema varies with the intensity of the wave- 

 lengths of those regions and the duration of exposure to the source 

 of ultraviolet rays, the distance from the source, the temperature, 

 and the individual sensitiveness of the skin. 



The dosage of light to be used therapeutically can not be fixed, 

 and it will vary with the disease treated. The sources of light and 

 the individuals irradiated vary too greatly to allow of any generaliza- 

 tion. Even different areas of the body vary greatly in their sensi- 

 tiveness to radiation. Some workers prefer an erythema, others a 

 suberythema dose ; still others strongly stress the desirability of the 

 production of pigment. The Copenhagen school, employing par- 

 ticularly the carbon arc light, prefers erythema. 



A practical method is to aim at a faint erythema production with 

 each dose applied. Thus skin sensitiveness is maintained. (This 

 dose of light has produced in the hands of some workers an in- 

 creased bactericidal power of the blood, whereas excessive dosage 

 has produced a marked decrease and in animal experimentation 

 has appeared to hasten death.) Overheating of the body is avoided 

 by using short and intense exposures. The degree of skin erythema 

 may guide in regulating dosage. Exfoliating skin is very opaque 

 to ultra-violet rays, while newly exposed skin is very sensitive. The 

 skin is rested during desquamation and several days elapse before 

 further exposure to this area. When the skin becomes insensitive, 

 say to large doses of mercury-vapor quartz light, an exposure to 

 long-flame carbon arcs is employed. When pigment is established 

 by this source, so that a long exposure is now required to produce 

 redness, this exposure can be reduced by using nickel-cored or tung- 

 sten-cored carbons. The method requires only 15 minutes' maxi- 

 mum radiation in contrast to the Finsen method, with which a 

 2-hour exposure is the rule.^° Skin colorimeters have been made 

 aiming to measure varying degrees of erythema ; but their accuracy 

 is questipned. Rollier and his followers, on the other hand, aim 

 for pigment production. 



Overdosage of light may produce injury, although nature has 

 left a wide margin of safety. Excessive exposure has caused a 

 drop in the bactericidal power of the blood with malaise and fatigue. 



so Eidinow, A., Med. .Tourn. and Rec, vol. 130, p. 695, Dec. 18, 1929. 

 149571—33—27 



