<Tr> <Th> Cause </Th> <Td> Heat inversion over East Coast </Td> </Tr> <Tr> <Th_colspan="2"> Casualties </Th> </Tr> <Tr> <Td_colspan="2"> <P> There are several estimates for the number of casualties caused by the smog: </P> <Ul> <Li> 168 (estimate from 1967 medical study) </Li> </Ul> </Td> </Tr> <P> There are several estimates for the number of casualties caused by the smog: </P>

Which of the following in usa faced problems of photochemical smog