Honestly the secondhand smoke research is very sketchy. Actually anything that becomes a public health issue ends up getting distorted by the public health establishment. Case in point: flu vaccines. They’re so ineffective that no rational person would ever waste time taking them, yet many of us do: https://www.cochrane.org/CD001269/ARI_vaccines-prevent-influ... (note: the “no rational person” is slight hyperbole)

The better argument is that the actual social behavior of smoking is contagious, as any social behavior is.