US researchers have identified a new biomarker which revealed that known and unknown exposure to second-hand smoke may lead to an increased risk of mortality in non-smokers.

Serum cotinine -- a metabolite of nicotine -- when used as a biological marker of exposure to second-hand smoke was found to have associations to overall and cause-specific mortality in non-smokers.

Increased levels of serum cotinine in blood were significantly also associated with all types of cancers, and heart disease, the researchers said.

"The study found that non-smokers are exposed to second-hand smoke without even realising it," said Raja Flores, Professor at the Icahn School of Medicine at Mount Sinai in New York, US.

Non-smoking individuals' cotinine blood levels accurately determined their exposure and subsequent risk of lung cancer and other smoking-related disease, Flores said.

"Using cotinine level to measure exposure to second-hand smoke has important public health implications, because increasing the scope of smoke-free environments would likely decrease cotinine levels in the general population and ultimately death," added Emanuela Taioli, Director at Mount Sinai.

Further, exposure to second-hand smoke is unequally distributed in the population, the researchers said, adding that children, people living in poverty, and those who rent their housing are disproportionately affected and most vulnerable.

The study, published in the journal Carcinogenesis, provides a more accurate way to gauge second-hand smoke exposure.

It also presents a strong case for more stringent limits on smoking and increased preventive screenings for those more likely to have been exposed to second-hand smoke.

For the study, the team examined 20,175 non-smokers. After adjustment for sex, education, race/ethnicity, body mass index, and smoking habits, their analysis showed a significant increase in years of life lost across cotinine concentrations.

In the adjusted analysis, the lowest quartile of cotinine concentration -- below the detectable level -- was associated with 5.6 years of life lost while the highest quartile was linked to 7.5 years of life lost.

A stricter legislation establishing smoke-free areas, together with education efforts in low-income and minority communities, is imperative, the researchers concluded.