Children who have been admitted to hospital with asthma are significantly more likely to be readmitted within a year if they are exposed to secondhand smoke in their home or car, a new study has found.

US scientists followed the progress of over 600 children aged between one and 16. They were attempting to determine the main reasons for readmission to hospital among children.

All of the children were first admitted to hospital in 2010 and 2011. The cotinine levels in their blood and saliva were measured. Cotinine is a by-product of nicotine that is found in the blood and saliva. It is often used as a measurement of smoke exposure.

The children's parents were asked about smoke exposure and all of the children were followed up for at least one year to see if any had to be readmitted at any point.

The study found that children who had been originally admitted for asthma were more than twice as likely to be readmitted to hospital within 12 months if they were exposed to secondhand smoke.

"The ability to measure serum and salivary cotinine levels presents the possibility of an objective measure that can be obtained when a child is seen in the emergency department or in the hospital and may be used to predict future hospitalisations," suggested the study's lead author, Dr Robert Kahn, of Cincinnati Children's Hospital.

Details of these findings are published in the journal, Pediatrics.

For more information on asthma, see our Asthma Clinic here