284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



pollutants as evaluated for 1954 [7]. Figure 11 represents the rela- 

 tive contributions of each of the four sources to the mean concentration 

 at California Institute of Technology as functions of the hour of the 

 day. It should be emphasized that these results cannot be directly 

 compared with measured mean concentrations at the institute. In 

 this example we have not, indeed, taken into account the chemical 

 reactions of the various pollutants that may occur before they reach 

 the point of measurement. We shall refer later to some chemical 

 reactions and include their effect in the model of atmospheric 

 pollution. 



ALLOWABLE POLLUTANT CONCENTRATIONS 



The purpose of atmospheric pollution control is to limit the al- 

 lowed maxima of pollutant concentrations in the atmosphere of an 

 urban or rural area. These maxima are determined by the individual 

 and combined characteristics of the principal pollutants and the 

 degree of inconvenience and damage that is to be tolerated. The ef- 

 fects of long exposure to low concentrations and the effects of imme- 

 diate exposure to high concentrations should be considered in relation 

 to damage to health, crops, and livestock, as well as to the deterioration 

 of property. It is necessary, therefore, to take into account the allow- 

 able long-exposure concentrations and the allowable threshold con- 

 centrations beyond which harmful and damaging effects will result. 

 A quantitative determination of the safe limits of concentration is 

 still an object of research studies. 



In determining allowable threshold concentrations, one must take 

 into account several factors involving not only the receiving end but 

 also the emitting origins of atmospheric pollution. Too stringent 

 limitations of allowable levels of pollution may impose great costs 

 and difficulties in operating many indispensable or useful activities 

 of the population. On the other hand, insufficient restrictions may 

 result in harmful effects on the health of the population or damage to 

 property. Therefore, the costs and operational difficulties of limita- 

 tions on the allowable levels of pollution should be measured by the 

 beneficial effects of such reductions. 



TEMPORARY EMERGENCY MEASURES 



One may wish to consider some temporary emergency measures 

 that would limit or modify the operation of the sources of pollution 

 and reduce the contamination. One could, for instance, consider 

 changing the hours of operation of the incinerators to shift their con- 

 tributions to pollution to other hours (see fig. 12). The contribution 

 of motorcars can be modified by changing, for the emergency period, 

 the traffic pattern or traffic regulations. As a result, idling and de- 

 celeration periods, during which the motorcars emit large amounts 



