What repels the Aedes mozzie

This story was first published in The Straits Times on Oct 17, 2013

Singapore is still in the grip of its worst dengue outbreak, with a record 17,686 cases reported this year.

In comparison, only 4,632 cases were reported last year.

The previous epidemic happened in 2005, when 14,209 people were infected.

Though dengue fatalities are uncommon, they can happen, especially as a result of a severe form of dengue called dengue haemorrhagic fever.

A woman died earlier this month, making her the sixth person to die of the disease this year. In 2005, 25 people died of dengue.

On top of this, there is also a chikungunya outbreak to avoid. Even though the numbers are much smaller than dengue, this disease was practically non-existent in Singapore several years ago.

Both are transmitted through the bite of an infected Aedes mosquito, which is active between dawn and dusk.

The symptoms - fever, chills, headaches, nausea, rashes and joint aches - are similar.

The differences: Chikungunya is rarely fatal but the joint pain that it causes can last for weeks or months.

Experts have said the best way to avoid either disease is to not get bitten by mosquitoes.

The best defence is to use a repellent correctly to ward off those pesky bloodsuckers.


Generally, repellents can be in the form of aerosol, liquid, cream or solid.

Look for the ingredients on the label before buying the product.

For repellents that are applied on the skin, those containing the chemical Deet (N,N-diethyl-m-toluamide) or another chemical called Picaridin as the active ingredients are most effective in repelling mosquitoes, said a National Environment Agency (NEA) spokesman.

People should apply a small amount of the repellent on the skin to test for allergic reactions, it said.

Deet, which has been commercially available since 1957, is now the most common active ingredient in repellent products.

Those who cannot tolerate Deet - it can cause rashes in rare cases - can use Deet-free repellents which are easily available in stores.

Deet is not suitable for babies under two months old and, other measures, such as the use of a mosquito net, should be employed instead, the NEA said.

It added that children should use repellents with lower concentrations of Deet, though it did not specify the amount.

Increasing the concentration of Deet does not make the repellent more effective; it just means the person will be protected for a longer period, the NEA said.

It has been documented that concentrations of about 6 per cent protects for about two hours and 20 per cent for about four hours, the agency said.

It has also been found that the period of protection plateaus around a concentration level of 50 per cent.

The American Academy of Pediatrics has advised against using products with more than 30 per cent Deet on children.

Going by this rule, complications arising from the usage of Deet should not be a worry here as the products sold here contain concentrations of between 7 per cent and 25 per cent, based on information from the NEA.

If you are still concerned, you can follow guidelines from Canada, which are stricter.

Products containing up to 10 per cent Deet may be used on children aged six months to 12 years, but should not be applied more than once a day on children below two years of age, Health Canada advised. It does not advocate the use of Deet on babies below six months of age.

Picaridin is a newer, odourless chemical that has been around since the 1980s, though it became available in the United States only in 2005.

As with Deet, do not use it on infants below two months of age.

Another chemical that can be found in insect repellents is Permethrin, but it is an insecticide and is intended for use only on clothing, and not on skin.

Whichever product you choose, follow the instructions on its usage.

Never apply an insect repellent on wounds or irritated skin.

Do not spray it directly on the face. Spray it on your hands and apply it to your face.

There is no need to go overboard, as applying more repellent does not increase its efficacy. Just be sure to cover all exposed skin and reapply when necessary.


If you do not wish to use chemical ingredients, you can opt for natural products formulated as essential oil, cream, patches or candles.

"Natural repellents, such as plant-based oil, may provide some short-lived protection, but frequent reapplication is required," said the NEA spokesman.

These natural products include lemon eucalyptus, citronella, cedar, peppermint, lemongrass, geranium and soya bean, he said.

"Of these, lemon eucalyptus has been shown to be the most effective," he added.

Generally, you need to reapply natural repellents every one to two hours.

But even natural repellents may not be completely safe.

For instance, you should not slather on essential oil. It is concentrated and can irritate the skin.

So, while studies have proved that oil of lemon eucalyptus is effective in repelling mosquitoes, it should not be used on children younger than three years of age, the Centers for Disease Control and Prevention in the United States advised.


Also available these days are devices that emit carbon dioxide and mild heat, similar to a human body, to attract the biting female mosquitoes into a trap that kills them.

But the effectiveness of these devices has not been proven, said Mr Ng Say Kiat, vice-president of the Singapore Pest Management Association.

"This is just a machine that generates heat. The mosquito may still prefer a person. There is more surface area so a person will generate a lot more heat than the machine," he said.

Other devices use ultraviolet light to attract mosquitoes into an electrocuting trap.

Mr Ng said these light traps are more for blue bottle flies and houseflies and may be less effective for the day-biting Aedes mosquitoes.

Other devices use ultrasonic technology to produce high-frequency sounds to repel insects.

"It has been confirmed that these are not effective," said Mr Ng.

In 2002, the Federal Trade Commission in the US found a manufacturer of ultrasonic devices to be making false and unsubstantiated claims. The commission said these devices do not kill or repel mosquitoes, or protect humans and animals from any mosquito-borne disease.

"These are gimmicky products. Some may work and some may not work at all," said Mr Ng.

"It is better to use repellents and take care not to breed mosquitoes at home. You can wear thick clothing if you want to go to forested areas."

The use of mosquito coils, which emit a repellent, can be of some use. Use electrical mosquito coils instead of the traditional kind, which has to be lit and can be a fire hazard, Mr Ng advised.

If you have no repellents or mosquito coils on hand, sit in front of a fan as the breeze will make it hard for mosquitoes to land on you.

This story was first published in The Straits Times on Oct 17, 2013

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