Kids with asthma are more likely to have breathing problems and be hospitalised when they live with a smoker, a research review suggests.
The team analysed data from 25 studies that included a combined total of more than 430,000 children.
Kids with asthma who were also exposed to second-hand smoke were 66 per cent more likely to seek emergency care and 85 per cent more likely to be hospitalised than their peers who didn't spend time around smokers, the study found.
For asthmatic kids, breathing in cigarette smoke was also linked to a more than tripled risk of poor lung function and 32 per cent higher odds of wheezing symptoms.
While the risk of smoke exposure exacerbating asthma symptoms is well known, fresh evidence on the extent of the danger posed to children may help convince some parents to abandon their cigarettes, said senior study author Dr. Avni Joshi, an allergist and immunologist at Mayo Clinic Children's Center in Rochester, Minnesota.
"There is hope that smoking cessation will help improve asthma symptoms and health care utilization even after any duration or extent of second-hand tobacco exposure," Joshi said by email.
"In addition, children learn from parental behaviour and they are less likely to start smoking themselves if they do not observe parental tobacco use."
Globally, an estimated 235 million people suffer from asthma, according to the World Health Organisation.
Asthma is an incurable chronic disease that leads to sudden attacks of breathlessness and wheezing.
During attacks, the lining of the bronchial tubes swells, narrowing the airways and restricting the flow of air in and out of the lungs.
Symptoms may be controlled with medication, but asthma can become life threatening when it isn't properly managed.
The causes of childhood asthma aren't clearly understood, but the condition has been linked to exposure to cigarette smoke and other air pollution as well as to certain airway infections that kids can get at a very young age.
Inhaled toxins can also trigger asthma attacks, as can certain viral infections, allergies, physical activity and changes in the weather.
One limitation of the current study is that most of the data also came from research in more affluent countries, making it possible that the findings might not apply to the developing world, the authors acknowledge in the Annals of Allergy, Asthma and Immunology. Researchers also lacked data on how the amount of second-hand smoke exposure impacted the odds of hospitalisation or complications.
Even so, given that there is no safe level of exposure to tobacco smoke, the study results aren't surprising, said Sam Oh, a tobacco researcher at the University of California, San Francisco.
"Tobacco smoke is a complex mixture of toxins, carcinogens and irritants," Oh, who wasn't involved in the study, said by email.
"Exposure to this brew causes immediate, measurable damage to our blood vessels and prevents them from functioning normally, including causing a child to have an asthma attack severe enough to warrant hospitalisation."
Families of children with asthma face significant barriers to keeping symptoms under control, said Dr. Annie Lintzenich Andrews, a pediatrics researcher at the Medical University of South Carolina in Charleston.
Parents may be juggling daily or even twice-a-day medications for their kids to prevent asthma attacks, while also worrying about environmental triggers such as pollen and dust mites, Andrews, who wasn't involved in the study, said by email.
If parents smoke themselves or know children spend a lot of time with a smoker, that just gives them yet another thing to worry about.
"It is incredibly difficult for parents to manage all of this on top of the stressors of everyday life," Andrews said.
"If a parent is struggling with smoking cessation, this study could provide that final reason they need to quit - no one wants their child to be hospitalised for a potentially preventable problem."