New Delhi, Oct. 20: An environmental think tank today questioned the practice of insecticide fogging by municipal authorities, claiming it is ineffective in controlling dengue and can harm the environment. But malaria control scientists have disputed its claim.

The Centre for Science and Environment (CSE) said fogging with insecticides knocks out adult mosquitoes and thus offers only "temporary control", and does not kill the larvae or the source of the next generation of mosquitoes.

The CSE, after examining fogging practices in Delhi which has this year has experienced its worst outbreak of dengue in two decades, has also pointed out that conditions such as wind speed and direction and air temperature can also affect the effectiveness of fogging.

"Unless repeated, fogging cannot control the next batch of adults out of the larvae, this is why source control through larvae is considered effective," said Amit Khurana, head of the food safety and toxins programme at the CSE.

But a scientist with the National Institute of Malaria Research (NIMR) said it would be misleading to claim fogging is completely ineffective. "Fogging can reduce the numbers of adult mosquitoes that spread infections," said Ranvir Singh, a scientist at the NIMR's field unit in Nadiad, Gujarat. "But it is not effective against larvae - other steps need to be taken," Singh told The Telegraph.

The CSE, in a media release issued today, has contended that fogging has a detrimental impact on human health because the process consumes diesel. A vehicle-mounted fogging container typically has 95 litres of diesel which is used up in one round of fogging, the CSE said.

A 100-day fogging-intensive season in the capital would use up 4,500 litres of diesel, the CSE said, adding that direct inhalation of diesel fumes, combined with insecticides, can exacerbate asthma or bronchitis among people with these respiratory illnesses.

Since the start of the monsoon season, the capital's municipal authorities have been fogging different neighbourhoods in the city which has this year documented over 12,500 patients with dengue, among whom more than 30 have died.

The CSE said the pattern of cases - 778 in August, 6,775 in September, and 4,925 in October - suggest fogging has no effect on the number of cases. "It only creates a false sense of security among people and diverts attention from preventive action," claimed Chandra Bhushan, CSE deputy director-general.

But a senior government scientist who specialises in mosquito control said fogging is useful during outbreak situations. "It can quickly reduce the density of mosquitoes in neighbourhoods," said the scientist who requested not to be named.

Larvae need to be tackled through larvicidal agents or biological control such as the use of larvae-eating fish, the scientist said.

Public health experts have long advocated a mix of measures to prevent and control outbreaks of mosquito-transmitted infections such as dengue, Japanese encephalitis, or malaria, including steps to curb the proliferation of potential breeding sites.