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warns against relying more heavily on programs focused on 

 determining individual source contributions. Such 

 programs are "extremely time-consuming and expensive" and 

 subject to "considerable uncertainties." 



o Scientific models to design the regional visibility 

 programs "are available today." The NRC Report 

 recommends that regulators use a nested progression, 

 employing the simpler and more direct models to design 

 regional visibility programs, and turning to the more 

 complex and detailed models to refine the programs over 

 time. 



o Important differences between the visibility problem in 

 the West and in the East may necessitate different 

 approaches. 



o Efforts to improve visibility in national parks and 

 wilderness areas will also improve visibility in other 

 areas. Such efforts could also help reduce certain air 

 pollution effects on human health and the environment 

 caused by the same pollutants. 



o Protecting visibility will require a "substantial, long- 

 term program." 



