Calcutta: Smoke fumigation during the dengue season will be kept to a minimum from next year, heeding warnings by public health experts that the harmful effects of this routine prevention activity far outweigh the benefits.

Fogging, colloquial for smoke fumigation, is for most people the one visible sign of the civic authorities doing something to combat disease-spreading mosquitoes.

But Atin Ghosh, the mayoral council member for health, said this would henceforth be done only when multiple dengue cases are reported in a particular locality. The practice of using smoke to fumigate the entire area within a 100-metre radius of a building where a dengue patient lives has been discontinued.

Civic health workers will instead focus on identifying and destroying mosquito breeding sites within a 100-metre radius of a house where someone has tested positive for dengue.

"By the time civic workers reach a particular area for fumigation based on information about a new dengue case, the (14-day) life cycle of an adult Aedes aegypti mosquito is over. The trick is to target new breeding places to prevent the mosquito population from growing," Ghosh said.

Debashis Biswas, chief vector control officer in the Calcutta Municipal Corporation, explained how "indiscriminate fogging" is a waste, besides being a health hazard.

"An Aedes aegypti mosquito becomes infective (ability to infect) seven days after it picks up the virus from a human. This mosquito then bites an uninfected human and transmits the virus," he said. "The person won't normally show the first symptom of infection, which is fever, until seven days after the bite. An NS1 blood test will show a positive result only after fever persists for at least two days. Another two days could be lost before civic workers get to know of it and turn up to fumigate the area."

In many cases, more than a fortnight passes between a larvae becoming an adult mosquito and a civic team reaching that area for fumigation.

Besides the futility of such an exercise, smoke fumigation is known to cause irritation in the eyes, allergies and respiratory trouble. The smoke that is emitted - resulting from the burning of a mixture of diesel and pyrethrum in the ratio 19:1 - is carbon dioxide. "It is never advisable to inhale smoke, especially by someone with a history of respiratory problems," said a doctor.

Fumigation might be effective only if there is "clustering", the medical term for several people being infected in a small area within a week or so. "A concentration of dengue cases indicates a sizeable population of mosquitoes. Fumigation can then be of use," a public health expert said.