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Sure, no problem....
http://www.surgeongeneral.gov/library/secondhandsmoke/factsheets/factsheet6.html
http://www.cancer.gov/cancertopics/factsheet/Tobacco/ETS
http://www.oehha.org/air/environmental_tobacco/netsexec.html
Concerning your question about "occasional breath of 2nd hand [sic] smoke" this is the summary from the Surgeon General's report:
There is no safe level of exposure to secondhand smoke. Studies have shown that even low levels of secondhand smoke exposure can be harmful. The only way to fully protect nonsmokers from secondhand smoke exposure is to completely eliminate smoking in indoor spaces. Separating smokers from nonsmokers, cleaning the air, and ventilating buildings cannot completely eliminate secondhand smoke exposure.
Supporting data is in the report.
How about bus fumes - is there a safe inhalable level? The guy worried about inhaling smoke while outdoors should be writing letters to his local officials about all those nasty bus fumes - I'm sure he's in some sort of immenent danger from diesel fuel combustion from improperly tuned bus engines. This gets funnier by the minute.