A new study has found that children's exposure to second-hand smoke has declined by approximately 80 percent since 1998.

Researchers collected and analyzed data from more than 35,000 children who participated in the annual Health Survey for England (HSE) from 1998 to 2012.

Their exposure to second-hand tobacco smoke was measured through the presence of cotinine, a nicotine derivative, in saliva samples. The body converts nicotine that has been absorbed as a result of inhaling tobacco smoke into cotinine which stays in the body much longer than nicotine and so provides an accurate record of the quantity of smoke inhaled in recent days.

In the late 1980s, the concentration of cotinine in the saliva of non-smoking children averaged 0.96 ng/ml. By 1998, that figure had dropped to 0.52 ng/ml, and by 2012 it had dropped further to 0.11 ng/ml. By 2012, over two thirds of all children had undetectable levels of cotinine, an occurrence that was once a rarity.

Researchers found this could be a result of an emerging social norm in England which has led to the adoption of smoke-free homes not only when parents are non-smokers but also when they smoke.

They found that the proportion of children living in a home reported to be smoke-free increased from 63 percent in 1998 to 87.3 percent in 2012.

Five years ago, the UK government included in its national tobacco control plan the ambition to see two-thirds of households with smoking parents go smoke-free by 2020. More than half of homes containing children with smoking parents have gone smoke-free. If current trends continue, government targets for protecting children from second-hand smoke will be reached ahead of time.

The findings are detailed in the journal Addiction.