An Australian research team has concluded that smoking is the greatest modifiable risk factor for sudden infant death syndrome (SIDS) also known as "cot death". Until now it had always been thought that babies sleeping on their tummies were the major factor.

The study showed that babies of smoking mothers had their "sleep arousal process", which is the reflex that wakes a person up in response to a life-threatening situation, changed. That meant that they were at higher risk of cot death.

The study included 12 healthy, full-term infants born to mothers who smoked an average of 15 cigarettes a day. The infants' arousal responses during daytime sleep were compared with those of 13 healthy infants of non-smoking mothers. All the children were assessed three times: at 2 to 4 weeks of age, 2 to 3 months, and 5 to 6 months. The risk of cot death drops off substantially for babies of 6 months or older.

The results showed that infants who had been exposed to smoke were less likely to wake up given certain stimulation, implying that they might not wake up in response to a life-threatening situation.

The study also identified a dose-dependent relationship between cortical activation (the process of "waking up") rates and levels of infant urinary cotinine, a nicotine metabolite. Infants with the highest levels of smoke exposure had the lowest levels of cortical arousal.

Senior investigator Rosemary Horne, scientific director of the Ritchie Centre for Baby Health Research at Monash University in Melbourne commented: "Our study suggests that maternal smoking can impair the arousal pathways of seemingly normal infants, which may explain their increased risk for SIDS."