Professor Philip Eng used to wonder why one of his patients, a 16-year-old girl, had such bad asthma attacks that she needed multiple hospital admissions even though he had put her on effective medication.

After reviewing the diagnosis, the senior consultant respiratory physician at Mount Elizabeth Hospital suspected that she might not be taking her medication dutifully.

The patient had consultations with him for about two years, when one day he began to notice that her asthma was getting much better.

At about the same time, he also noticed that it was her grandfather who was taking her for her check-ups.

Previously, it had been her mother.

After finding out that both her parents were smokers, he put two and two together. He asked some questions and found out that the girl's parents had been posted to China for one year.

She was now being looked after by her grandparents, who did not smoke. Within a month, her asthma had become better and she did not need to take all of her daily medication. She had been a victim of passive smoking.

The harm from passive smoking was first studied by Japanese researcher Takeshi Hirayama in 1981. He found a link between lung cancer in non-smoking Japanese women and their husbands who were smokers.

The link has since been confirmed by many more studies and new scientific evidence. A passive smoker breathes in sidestream smoke from the burning tip of the cigarette, and mainstream smoke inhaled and then exhaled by the smoker.

The risk of lung cancer in passive smokers is increased by between 20 and 30 per cent. The added risk of heart disease is 23 per cent.

Children are especially at risk from adults' smoking.

Adverse health effects include pneumonia and bronchitis, coughing and wheezing, worsening of asthma and possibly neuro-behavioural damage and cardiovascular disease in adulthood.

For the sake of their children, smoker parents should think about quitting the habit.

Caregivers should also be non-smokers.

Smoking outdoors does not safeguard children from passive smoke.

A study by Curtin University of Technology published in June 2008 found that even if parents smoked only outside, the levels of respirable suspended particles, including nicotine, were still much higher in the homes.

Boston researcher Jonathan Winickoff has coined the term "third-hand smoke".

It is the cocktail of toxins that linger in carpets, sofas, clothes and other material hours or even days after a cigarette is put out.

There is help here if you want to quit.

You can call the QuitLine on 1800-438-2000, where professional quit advisers will guide you. Alternatively, you can join the smoking cessation sessions offered at polyclinics and hospitals.

This article was first published in Mind Your Body, The Straits Times.