Tobacco smoke harms only the person actively smoking it.

Once the dangers of active smoking became clear, involuntary exposure still seemed like a different problem. A nonsmoker in the same room inhaled a much smaller dose, and ventilation or dilution was often assumed to make that dose medically insignificant.

Epidemiology showed otherwise. Studies of nonsmokers living with smokers found elevated lung-cancer risk, and later research linked secondhand smoke to coronary heart disease, stroke, respiratory illness, and harms in infants and children.

Biomarkers confirmed that nonsmokers were absorbing tobacco-smoke chemicals rather than merely smelling them. Smoke-free workplace and public-space studies added evidence that reducing exposure produced measurable health benefits.

Secondhand exposure is generally lower than active smoking, so the risks are not numerically identical. Lower exposure, however, does not mean zero biological effect.

Ordinary ventilation, air cleaning, or separating smokers within the same indoor space cannot reliably eliminate exposure.

The correction is therefore about causation, not equivalence: secondhand smoke is less concentrated than actively inhaled smoke but still causes disease.