Reference summary — not medical advice or a personal risk estimate.
Secondhand tobacco smoke
Quick answer
Classified as Group 1 — carcinogenic to humans, based on sufficient evidence in humans. Breathed. Breathing other people's tobacco smoke indoors — in homes, vehicles, and workplaces without smoke-free policies.
Summary
Breathing other people's tobacco smoke is associated with lung cancer, even for nonsmokers.
Detail
Secondhand smoke is Group 1 with sufficient evidence for lung cancer. WHO, IARC and major national cancer bodies agree there is no safe level of exposure. Smoke-free home and workplace policies are the most effective protection.
What's not established
This classification is based on sufficient evidence in humans that this factor can cause cancer. It does not establish a safe exposure threshold, your personal risk, or how this factor's risk compares in size to other Group 1 agents — hazard classifications are not ranked by magnitude.
Regional relevance
global
Documented in these products
Presence in a product category doesn't imply the product causes cancer. Click each to see the source and context.
Evidence sources
Every claim on this page traces back to one of these. Click through to read the primary document yourself.
Primary sources
NCI
NCI — Risk Factors: Tobacco
NCI
NCI — Cancer-Causing Substances in the Environment
IARC
IARC Monographs — Classifications by Cancer Site, Vols 1–140
Supporting sources
Canadian Cancer Society
Canadian Cancer Society — Reduce your risk
ACS
ACS — Known and Probable Human Carcinogens
Cancer Research UK
Cancer Research UK — Causes of cancer and reducing your risk
IARC / WHO — European Code Against Cancer
European Code Against Cancer (5th edition) — 14 ways you can help prevent cancer
NTP
NTP — Report on Carcinogens (US National Toxicology Program)
Cancer Council Australia
Cancer Council Australia — Causes of cancer
Last reviewed: October 2026
Official reference list
A government list some readers use alongside the primary evidence. Meyron is not affiliated with it. A chemical being listed does not by itself describe a risk from any particular product.
- California Prop 65 list
State of California's list of chemicals known to cause cancer or reproductive harm.