With the arrival of rainy summer, mosquitoes start to breed profusely, and as a matter of course it is crucial to take adequate precautions against mosquitoes while enjoying nature and outdoor activities. The Consumer Council tested 25 models of mosquito repellent on humans and found that nearly 90% of models containing DEET or picaridin could still provide 80% or more protection 4 hours after application, among which 1 DEET containing model was the only that had no mosquitoes landing on the tester’s forearm, thus providing the most protection among all. Despite the high repellent efficacy, consumers should be aware that a high concentration of DEET may irritate skin or cause rashes and blisters. On the other hand, some models labelled as “natural” or “herbal” were quite effective in repelling Culex quinquefasciatus (commonly known as “southern house mosquito”), known to transmit filariasis, but less effective in repelling Aedes aegypti (commonly known as “yellow fever mosquito”), known to transmit dengue fever. 1 model showed more mosquitoes landing on the tester’s forearm after use, indicating that it might contain ingredients that attract mosquitoes. Consumers are advised to assess the risk of mosquito-borne diseases at their destinations and estimate the duration of stay before choosing suitable mosquito repellents, and to apply the products according to instructions on the labels.
The Council purchased 25 different mosquito repellents from various retail outlets. 5 were labelled containing DEET, 4 with picaridin, 2 with ethyl butylacetylaminopropionate (IR3535), 3 with p-menthane-3,8-diol (PMD), 1 with cedar oil, 2 with wild tomato extract, and other 8 models claiming to be “natural” or “herbal”, ranging from $18.9 to $259 per bottle.
The test was conducted in Australia and was designed with reference to the World Health Organization’s “Guidelines for efficacy testing of mosquito repellents for human skin”, using the “arm-in-cage” laboratory method to test mosquito repellent efficacy on real people against southern house mosquito, known to transmit filariasis, and yellow fever mosquito, known to transmit dengue fever. The mosquito repellents were individually applied to testers’ forearm skin, and the number of mosquito landings on treated forearms was recorded at 4 different time points after application, namely 0.5 hours, 1 hour, 2 hours and 4 hours, in comparison with the number of mosquito landings on forearms without repellent application. The higher the calculated percentage, the better the performance of a particular mosquito repellent.
DEET Offers Strongest Protection but High Concentration May Irritate Skin
DEET has long been widely used in insect repellent, while picaridin, both colourless and odourless, and does not make the skin feel sticky, is also commonly found in mosquito repellent products. For the 9 models labelled as containing these 2 ingredients, overall mosquito repellent efficacy was high for both southern house mosquito and yellow fever mosquito. For southern house mosquito, 9 models still provided 96% or more repellent efficacy 4 hours after application, among which 7 models even reached 100%, showing a stable performance. For yellow fever mosquito, except for 1 model labelled with a lower DEET concentration (6%), repellent efficacy of which dropped to 35% 4 hours after application, all other models maintained 80% or above protection. 1 model labelled with DEET was the only among all 25 tested models that had no mosquito landings on the tester’s forearm at all time points of the test, offering the strongest protection.
Performance of the 2 models labelled with IR3535 varied. 1 model, for either type of mosquito, maintained over 84% repellent efficacy at all 4 time points; the other model, although having 100% repellent efficacy against southern house mosquito, its efficacy against yellow fever mosquito dropped to 49% after 4 hours. Of the 3 models labelled as containing PMD, 2 performed very well against southern house mosquito, while performance of the remaining 1 dropped to 49% after 4 hours; against yellow fever mosquito the efficacy of these 3 models decreased gradually over time, from 97%, 82% and 73% after first 30 minutes to 76%, 74% and 30% after 4 hours respectively. 1 model which was labelled as containing cedar oil, its repellent efficacy dropped to 53% only after 2 hours. For the 2 models labelled as containing wild tomato extract, repellent efficacy against yellow fever mosquito was poor, with only 16% and 26% efficacy respectively 30 minutes after application, and declining further over time.
Although DEET has the highest repellent efficacy among these ingredients, consumers should be aware that the use of high concentration of DEET products or excessive exposure to DEET may cause rashes, blisters, and irritation of skin and mucous membranes, and the risk to children is even higher. There are different recommendations on the use of DEET products on children in different regions. With reference to the recommendations of the Centre for Health Protection (CHP) of the Department of Health, children should use mosquito repellents at lower DEET concentration (up to 10%). Infants under 6 months of age should not use products containing DEET, unless they are travelling to countries or areas where mosquito-borne diseases are endemic or epidemic and exposure is likely, then children aged 2 months or above can use DEET-containing insect repellents with a concentration of DEET up to 30%. As for picaridin, which also proved to be more effective in repelling mosquitoes, although less irritating to the skin than DEET, it may still irritate the eyes, and the Department of Health in Western Australia does not recommend the use of this type of mosquito repellent for children under the age of 12 months, whereas Health Canada states that it should not be used on infants under the age of 6 months.
Natural Ingredients More Effective against Culex Quinquefasciatus
1 Model Attracted Instead of Repelling Mosquitoes
People who are allergy-prone or prefer natural ingredients are easily attracted by products labelled as “natural” or “herbal”. In this test, there were 8 other models which claimed to be natural or were labelled with botanical ingredients, such as citronella oil, lemongrass oil, soybean oil, lemon eucalyptus oil, etc. On the whole, efficacy of these products against southern house mosquito was satisfactory, with 7 models still attaining an efficacy rate of 70% or above at the 4th hour mark. However, protection of all these models against yellow fever mosquito was unsatisfactory, with 5 models having an efficacy of 20% or below 30 minutes after application, and all 8 models having an efficacy of below 30% after 4 hours.
It is worth noting that for 1 model, during tests on both types of mosquitoes, on average at most time points more mosquitoes landed on testers’ forearms after application than without, suggesting that certain ingredients contained in these models may be attractive to mosquitoes, luring them to land if left on the skin.
Although natural ingredients may pose relatively lower risk to the body, it does not guarantee absolute safety. For example, citronella oil may mildly irritate the skin, eyes, and throat, and prolonged or frequent contact may cause allergic reactions to the skin, whereas lemongrass oil contains the fragrance allergens geraniol and limonene. As repellent efficacy and safety of natural herbs vary, consumers are advised to refer to the instructions on product labels and exercise caution when using them, especially on young children and pregnant women.
Manufacturers Urged to Improve Unsatisfactory Product Labelling
As different ingredients have varying effects and risks to consumers, transparency and accuracy of product information is crucial, but labelling of some models was found to be unsatisfactory. 1 model had a list of ingredients on the bottle different from the one on the box, and some models did not provide a detailed list of ingredients other than the active ingredient. Some models neither explicitly stated whether they were suitable for children, nor mention suitable ages even if labelled as such. The Council stresses that manufacturers have a responsibility to clearly specify ingredients contained in their products, the age at which they are suitable for use, and the appropriate amount and method of use for each age bracket. At present, the Pesticides Ordinance in Hong Kong does not cover mosquito repellent products directly applied on the human body. The Council hopes that the authorities will refer to legislation in other places and impose appropriate regulation on the active ingredients, concentration, labelling, performance claims, applicable groups and age of mosquito repellent products directly applied to the human body, so as to safeguard consumer safety.
Mosquito repellent products with chemical or natural ingredients each have their own advantages and drawbacks. Consumers may consult physicians and pharmacists and choose suitable products according to usage and health conditions, and may refer to the following recommendations:
- To apply mosquito repellents, especially those containing DEET, picaridin, etc. on children, pay heed to whether concentration of relevant ingredients is appropriate, and apply following instructions on labels;
- According to the results of this test, most models containing wild tomato extract, claiming natural, or labelled containing botanical ingredients have good efficacy in repelling southern house mosquito commonly found in Hong Kong. However, to travel to areas where mosquito-borne diseases (e.g. dengue fever, Zika virus infection, etc.) are prevalent, it is advised to choose products containing DEET, picaridin, IR3535 and PMD which provide stronger protection;
- Mosquito bites should be kept clean, and wash the affected area with soap and water if possible. If redness, swelling, or itching occurs, apply an ice pack for a few minutes to relieve symptoms. Do not scratch the area to avoid bacterial infection;
- Avoid going out in the countryside at dawn or dusk when mosquitoes are more active, and wear light-coloured, long-sleeved shirts and trousers to minimise exposed skin. If you are out in the countryside during the day, apply sunscreen first and allow 15 to 30 minutes for it to be fully absorbed and dry before applying mosquito repellent.
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