In this study, we experimented on the effectiveness of 4 different types of repellent to repelling mosquitoes. Using the logic that a stronger repellent will repel more mosquitoes, and hence result in less bites, it was found that the order from strongest to weakest repellent is 'OFF', 'Citronella oil', 'MOZAWAY' and 'Peppermint oil'. Out of the 4 repellents, 'OFF' and 'MOZAWAY' are synthetic repellents, while 'Citronella oil' and 'Peppermint oil' are natural repellents that are known to have repelling effect on mosquitoes.
In each experiment, there will be a 'control' hand and an 'applied' hand. The 'control' hand is not applied with any of the repellent tested for each experiment whereas the 'applied' hand is the one with the repellent. On each hand, an area of 17cm x 10cm is drawn out, which served as the area to be observed for any mosquito bites. Repellent was applied within the designated area.
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In total, 4 trials were conducted with each repellent and the total number of bites for each trial in the designated area was recorded. In each table, the total number of bites from all 4 trials is tabulated.
Finally, the percentage effectiveness is then calculated from the result. The percentage effectiveness is calculated as follow:
Percentage effectiveness: 100%-[percentage ineffectiveness]:
=100%-[applied/total x 100%]
= [control/ total x 100%]
The rationale for this is that taking the percentage of bites when repellent is applied to be ineffective [applied/total], percentage effectiveness would then be taking [control/total x 100%]. Higher percentage effectiveness would means that the repellent has a stronger repelling effect.
From the calculated values of the percentage efficiencies of the various repellents, we can conclude that 'OFF' is a stronger repellent than the rest of the repellents tested, with a high percentage of '89.48%'.
This is because 'OFF' contains 25% N,N-Diethyl-meta-toluamide (DEET), which is proven to be a strong repellent compound. This is followed by the100% Citronella oil, a known natural repellent and thirdly, 'MOZAWAY', which contains 15% Citronella oil. Lastly, Peppermint oil is the weakest repellent as reflected in table 1, where there were more bites when it was applied.
The main attractant to mosquitoes found on human are odours in human breath and sweat, including carbon dioxide and lactic acid. In the mosquitoes' olfactory system, there are different receptors to detect these odours.
DEET works by interfering with the odorant receptors, so that mosquitoes are unable to detect their prey through the secretion from the body. This helps to make prey 'invisible' to the mosquitoes. This is in accordance with our result, as when using 'OFF', we accumulated the least number of bites, which reflects that DEET is indeed a very strong repellent. However, it is toxic when used in large amount. Upon research, it was also found that 100% protection against mosquito bites can be achieved when DEET-based repellents are used with Permethrin-treated clothing, where Permethrin is a widely used chemical in insecticide. Supporting proof included that 'OFF' emerged as the best repellent, with its effect lasting for 5 hours. Future research can be done on determining the amount of DEET in repellent to manufacture an effective mosquito repellent that deter mosquitoes which transmit deadly diseases, without toxic reaction when applied on the skin. Some people may experience rashes, blisters and even seizure if too much DEET (50-75%) was used.
100% Citronella Oil, having a percentage effectiveness of 55.0%, rates as second most effective repellent in our experiment. Since effectiveness of both natural repellent (Citronella oil and Peppermint oil) rates lower than 'OFF', it can be argued that plant-based repellents (Citronella and Peppermint oil) are generally less effective than DEET-based products (OFF).  This is supported with a research from University of Florida that DEET concentration at 23.8% provided 37.6minutes of protection while citronella-based lotion was only effective for 7.9minutes.
To be an effective repellent, it must show a substantial degree of volatility to deter mosquitoes. Citronella and peppermint oil, being volatile liquid, vaporises easily so the smell is quickly dispersed once applied. However, the effective repellent vapour concentration cannot evaporate so quickly at the surface of the skin that it loses its effectiveness. The smell from the vapour of the essential oil is not pleasant to the mosquito, acting as a repelling factor. With Citronella oil having a higher effectiveness percentage, it can be argued that Citronella oil has a higher volatility than Peppermint Oil and it is better at maintaining surface effectiveness. These results may explain why Citronella oil is found in most repellents that are DEET-free. An example would be "MOZAWAY' which contains 15% of Citronella oil. Further research could be done in this field to investigate the effect of 100% Citronella oil combined with other good repelling agents so that the problem of toxicity in DEET repellent can be overcome, without compromising on the effectiveness of the repellent.
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Lastly, 'MOZAWAY' was ranked third effective. It contains only 15% of citronella, which is considered diluted; hence the effectiveness was greatly reduced. For 100% citronella oil, there is a need to reapply every 30-60minutes because the volatile liquid vaporise easily. Therefore, the 'MOZAWAY' is comparatively weaker as the diluted solution will vaporise faster than a 100% Citronella Oil. This provides useful information to consumers so that they can choose a higher concentration of the active natural repellent if they opt for DEET-free repellents.
- Mathias Ditzen, Maurizio Pellegrino & Leslie B. Vosshall* (2008, March 14). Chemicals Like DEET In Bug Spray Work By Masking Human Odors.
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- Permethrin (n.d.). In Wikipedia. Retrieved March 18, 2010, from http://en.wikipedia.org/wiki/Permethrin.
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