Ear infections, a scourge that has left countless tots screaming through the night, have fallen dramatically, and some researchers suggest a decline in smoking by parents might be part of the reason.

Health officials report nearly a 30 per cent drop over 15 years in young children's visits to the doctor for ear infections. On average that's half a million fewer trips to the doctor.

Why the numbers are declining is a bit of a mystery, but Harvard researchers think it's partly because fewer people smoke, meaning less irritation of children's airways. Many doctors credit the growing use of a vaccine against bacteria that causes ear infections. And some think increased breast-feeding is protecting more children.

For decades, they were the most common reason parents brought young children to a doctor, according to health officials. The Centres for Disease Control and Prevention hadn't bothered to issue a report on them in nearly 20 years.

Cases skyrocketed from 1975-1990 when the rate for children 5 and under more than doubled in that time.

A big reason, Rosenfeld said, was a steady rise in dual-career families. More families put their kids in day care, and day care is a breeding ground for the germs that lead to ear infections.

But the study by Harvard University suggests another contributor: cigarette smoke.

Most ear infections occur after a cold. In children, the ear is more directly connected to the back of the nose, so infections in a child's nose and throat can easily trigger ear inflammation. Such swelling is a fertile setting for the bacteria that cause ear infections.

Cigarette smoke, inhaled through a child's nose, can trigger the same kind of irritation and swelling, said Dr. Gordon Hughes of the National Institute on Deafness and Other Communication Disorders.