[int_19h]

Logical in a sense that it's a reasonable initial premise in the absence of other evidence. Not saying that it's true, but if you don't know if it's true or not, and you know that smoking other things does give you cancer, that's the side I'd err on.

[dekhn]

nope, the null hypothesis is "smoking something does not give you cancer". That smoking other things gives you cancer is specific to what they contain.