[A_D_E_P_T]

> Changing your chances of contracting terminal cancer in the next 5 years from 1% to 0.5% is still significant.

Short of quitting smoking, there's almost no way to do that, because you don't know what the factors are or how they're weighed & influenced by individual genetic, immune, and metabolic factors.

> "How much do the controllable factors actually change the odds?"

Not only does nobody know, nobody has the foggiest notion. I've never seen a relevant population-wide observational study that wasn't so full of methodological flaws that you could spend hours picking apart its defects.

Of course we know that there's a set of factors associated with the development of cancer -- like, e.g., long-term exposure to polonium in drinking water -- but there's also a large set of factors that nobody is aware of or which can never be controlled for. To use only the first set -- the set of what we know and can control for -- to accurately estimate the odds of contracting cancer within the next five years is impossible.