Does passive smoking (secondhand smoke) increase your risk of cancer?
Passive smoking increases your risk of cancer, with the strongest evidence for lung cancer. The risk is real but smaller than with active smoking; avoiding exposure as much as possible is advisable.
Passive smoking increases your risk of cancer. A meta-analysis of 40 studies shows that significant exposure raises the overall cancer risk in never-smokers by approximately 16%. This effect was consistent across both case-control studies and prospective cohort studies.
Most of the evidence concerns lung cancer. Never-smoking women with a smoking partner have, on average, roughly 24 to 26% greater odds of developing lung cancer. A separate meta-analysis of 41 studies from the period 1997-2017 finds an increased risk of 33% when all study types are pooled together. However, when only the cohort studies are examined, that figure is smaller and is no longer statistically significant. A dose-response relationship has been demonstrated: the longer the exposure, the higher the risk, which strengthens the suspicion of a causal relationship. One caveat: a large cohort study among more than 323,000 Chinese never-smokers found NO significant association between passive smoking and lung cancer mortality. Indoor combustion of wood or coal was, however, a risk factor in that study.
For other types of cancer the picture is less clear. With prolonged exposure of more than 15 years, at home or in the workplace, there is an elevated risk of throat and laryngeal cancer. For cervical cancer there is an association, but it weakens when HPV infection is taken into account. For most other cancers, such as stomach, bowel, liver or bladder cancer, no convincing relationship has been found.
A large analysis published in Nature Medicine in 2024 added further nuance. That study rates the evidence for heart disease, stroke, diabetes and lung cancer as 'weak, two stars': real associations, but more uncertain than is generally presented. For breast cancer a statistically significant elevated risk was found (OR approximately 1.24), but that rating fell back to just 'one star', meaning the true increase in risk is likely smaller than the figure suggests.
In summary: the strongest and most consistent evidence applies to lung cancer in never-smoking women. The effects on other types of cancer are mostly small or uncertain. Nevertheless, no safe lower threshold for exposure is known, and less exposure is always better.
Based on meta-analyses, a prospective cohort study (n>323,000) and a pooled analysis of case-control studies, plus a Burden of Proof analysis (Nature Medicine 2024). Risk estimates vary by cancer type and study design. Publication bias and incomplete correction for confounding factors are recognised limitations in the field.
Does sitting a lot increase my risk of cancer, even if I exercise?
Sitting a lot increases your risk of certain cancers, even if you exercise. In addition to moving regularly, try to reduce your sitting time itself, because the two only partially compensate for each other.
Does being overweight increase my risk of cancer, and if so, which types?
Excess weight increases your risk of at least 13 types of cancer, including uterine, colorectal, breast and liver cancer. Losing weight reduces that risk, although it remains unclear whether all methods of weight loss are equally effective for immune defence against cancer.
Does poor sleep increase my risk of cancer?
Poor sleep, and certainly untreated sleep apnoea, is associated with an increased cancer risk, but it has not yet been proven that sleeping poorly causes cancer. Have sleep apnoea checked if you have symptoms, and try to maintain a regular sleep schedule.
Does working with pesticides increase your risk of cancer?
Working with pesticides is associated in multiple studies with an increased risk of several types of cancer, including bladder, bowel, and blood cancers. Avoiding unprotected contact is sensible, especially if you are pregnant or planning to become pregnant.
Does working at night increase my risk of cancer?
Night work probably increases the risk of breast cancer, and possibly of other types of cancer. If you have been working night shifts for years, it is worthwhile to discuss this with your doctor and to stay alert to population screening programmes.
Does exercise lower my risk of cancer?
Regular physical activity is associated with a clearly lower risk of several cancers in large population studies. Alongside that, aim to reduce the number of hours you spend sitting during the day, and use sun protection if you do a lot of outdoor sport.