Answer:
Approximately 3,000 lung cancer deaths occur each year among adult nonsmokers in the United States as a result of exposure to secondhand smoke.
SOURCE:http://www.cancer.gov/cancertopics/factsheet/Tobacco/ETS
*********
The true answer is that they have absolutely no idea. The number is generated by a computer model (GIGO - Garbage In, Garbage Out!). It's the same for 'smoking related', 'alcohol related' and 'obesity related' deaths.
Let's just say that the numbers generated by these computer models could be heavily subject to political and corporate influences.