People exposed to secondhand smoke have higher levels of cadmium – a toxic metal linked to cancer – in their bodies, a new study suggests.
Researchers found that adults with heavy passive smoke exposure had around 1.5 times more cadmium in their blood than those in smoke‑free environments, although the study did not assess whether this led to higher cancer rates.
Levels were still far lower than in active smokers, who had more than three times the amount.
High or long-term cadmium exposure has been linked to kidney failure, brittle bones and chronic breathing problems such as bronchitis and asthma.
Researchers assessed cadmium levels in the blood and urine of 1,380 children and teenagers and 3,686 adults using national health data from 2015 to 2020.
To determine recent tobacco smoke exposure, the researchers measured the levels of cadmium and nicotine in the subjects’ systems and categorised them as no exposure, light exposure, heavy exposure and active smoking.
Both blood and urine were measured. This is because blood is a highly effective tool for determining recent exposure to cadmium, while the kidneys retain cadmium for up to 30 years, making urine an accurate record of any long-term accumulation of cadmium.
Because the study is observational, it cannot prove that secondhand smoke directly causes cadmium buildup or cancer risk.
People exposed to secondhand smoke have higher levels of cadmium – a toxic metal linked to cancer – in their bodies, a new study suggests
Lead author Nandita Sarker said: ‘We knew that cigarette smoke exposes people to cadmium but until now, we didn’t know about the association with secondhand smoke.
‘This is important information because cadmium builds up in the body over time and is a factor in kidney, lung and prostate cancers.’
The study, published in the journal Biological Trace Element Research, found that across all age groups, women consistently exhibited higher cadmium levels than men.
This is because, the study explains, the female digestive tract naturally absorbs cadmium much more efficiently than the male digestive tract. This is even more the case during major hormonal shifts like menstruation, pregnancy and menopause.
The study also found that people in a racial minority group or who have lower income or less education face significantly higher exposure to cadmium, which may also reflect differences in diet or environmental exposure.
‘This disparity cannot be explained by smoking habits alone, but likely is the result of broader social, environmental and financial inequalities,’ Sarker said.
‘In these cases, the cadmium typically comes from crowded, multi-unit housing where smoke spreads through shared ventilation systems or from food, soil and traffic exhaust.’
‘Our findings suggest that secondhand smoke may contribute to the long-term accumulation of cadmium, a toxic metal associated with cancer and other chronic diseases,’ said Taehyun Roh, with the Department of Epidemiology and Biostatistics, who supervised the study.
‘These results highlight the importance of protecting people from tobacco smoke exposure not only for respiratory health but also for reducing exposure to harmful environmental contaminants that can accumulate in the body over time.’
However, the study is limited because cotinine – the primary breakdown product of nicotine – only stays in the human body for about 15 to 20 hours.
This short window means a single lab test cannot distinguish between someone who just walked out of a heavily smoke-filled room and an occasional active smoker, nor can it track dietary cadmium intake over several decades.
Secondhand smoke contains a cocktail of more than 4,000 irritants, toxins and cancer-causing substances.
Previous studies have found that people who breathe in secondhand smoke regularly are more likely to develop many of the same diseases as smokers, though generally at lower risk.
Secondhand smoke exposure is estimated to contribute to tens of thousands of child deaths globally each year.
The findings come as policymakers attempt to further reduce exposure to secondhand smoke.
In an effort to curb the risks of passive smoking, the Tobacco and Vapes Bill, which is intended to come into force on January 1, 2027, will make smoking or vaping in cars with children inside, in playgrounds and outside schools illegal.
Smoking will also be banned outside hospitals and GPs, though vaping will still be allowed in attempts to support those trying to quit.
Outdoor spaces such as pub gardens, beaches and private gardens will be exempt from the ban.
