Mosquitoes seem to have hit the city with a vengeance. According to figures released by the government, vector-borne diseases such as dengue, malaria and Japanese encephalitis have taken 26 lives in Calcutta. However, media reports claim that the figure is higher, 33.

As the incidence of malaria and dengue rises, so does the sale of mosquito repellents. Whether it is coils, liquid vapourisers, lotions, creams or aerosols — repellents of all kinds are selling like hot cakes. “The sale of major brands of mosquito repellents has increased by about 15-20 per cent over the past month,” says Samar Singh Sheikhawat, vice-president, marketing, Spencer’s Retail.

That may be good news for retailers, but consumers need to be aware that mosquito repellents can have certain potential side effects. Medical experts warn that excessive use of these products for long periods is bad for health. “Nervous weakness is an important side effect of chemicals used in mosquito repellents,” says Dr Pulak Lahiri, Nilratan Sarkar Professor of Zoology at Calcutta University.

“According to the National Occupational Health and Safety Commission, the US, long-term use of Alithrin and Prallethrin, the two major chemicals in these products, are hazardous to human health. They primarily affect our respiratory and immune systems, and in extreme cases, even our nervous system,” he adds.

Most health experts echo Dr Lahiri’s concern. “Smoke-based mosquito repellents such as coils can trigger wheezing attacks, specially in those who are prone to allergies and asthma,” says Dr Narayan Bhattacharya, a city-based general practitioner. “Coils and aerosol sprays are the worst offenders and their toxic fumes should be strictly avoided.”

Naturally, companies that sell mosquito repellents insist that their products are risk free. “The chemicals are present in such small quantities that they have an effect only on mosquitoes. Coils and aerosols cut off the oxygen supply for mosquitoes in the upper layers of the air, causing them to become inactive,” says Subrata Chakraborty of Reckitt Benckiser, which markets mosquito repellents under the brand name Mortein. “They do not harm humans if used in a well-ventilated room,” he asserts. He also mentions that Mortein carries a toll-free number that a consumer can call to lodge a complaint if he or she suffers any health problem on use.

Officials at Godrej Sara Lee, the makers of Good Knight mosquito repellents, also stress that if used properly, mosquito repellents do not have any side effects. “If consumers adhere to the directions of use as mentioned on the labels, there should not be any problem. For instance, one should always use liquid vapourisers with the windows open, and avoid excessive use of lotions and creams,” says an official.

However, most users do not buy the “proper use” argument offered by manufacturers of mosquito repellents. Says Arpita Bose, a Calcutta-based mother of two, “What is the point of using a vapouriser if the windows are kept open? It would only mean inviting more mosquitoes to come in,” she says. “Moreover, if the chemicals in coils and aerosols are cutting off the oxygen supply in the upper layers of the air, that is bound to affect human respiration too,” she points out.

But then should we avoid mosquito repellents altogether? “Certainly not,” says paediatrician Dr Santanu Ray. “At least repellents are much less damaging than dengue or malaria. But they do cause adverse reactions in children who are prone to allergies. Coils should be avoided in the case of children who are less than a year old as they are vulnerable to respiratory problems.”

Experts believe that excessive use of mosquito repellent creams and lotions too must be avoided. “It is a myth that these substances remain on the skin and so can’t have any adverse effects. Actually, lotions eventually get absorbed into the body and enter the bloodstream. So the safety of these products is also questionable,” says Dr Lahiri.

Studies on mosquito repellents have revealed that products like Deet, once used in countries such as the US, contained an active ingredient called N-N-diethyl-meta-toluamide that was reported to cause headache, nausea and seizures. In India some mosquito repellents contain N-ethyl benzamide, another form of the chemical used in Deet. However, there has been little research on its effects on humans.

So is there no product that will drive mosquitoes away and yet have no harmful effects on humans? Experts say that herbal mosquito repellents could be the answer.

“In countries such as Kenya, where malaria is a huge killer, citronella oil is used to keep away mosquitoes. Also, there are products available in European and African countries that use a World Health Organisation-approved formulation made from 14 medicinal plants and essential oils, like pine oil and eucalyptus oil,” says Dr Anirban Ghosh, a physician based in the UK. Anti-mosquito patches with citronella oil and mosquito repellent products that contain garlic and clove extracts are also available in other countries, he adds.

Unfortunately, herbal anti-mosquito products like these are not available in India. Scientists at the Centre for Rural and Cryogenic Technologies at Jadavpur University are now developing a candle containing neem and eucalyptus extracts. The candle is supposed to keep away mosquitoes for at least seven hours. But the product is still at an experimental stage.

So do use mosquito repellents if you have to. Just make sure that you take adequate precautions and find out if the product you are using is safe for you and your family.