Insect Repellent & Sun Block: which goes first?
If you’re in a country that requires you to apply sunblock, there’s a good chance you’ll also need to apply insect repellent.
There are many old traveller’s tales warning if you apply them in the wrong order you'll neutralise one, the other, or both!
A bit like the ‘milk in first?’ debate for tea drinkers, the travel medicine community has been discussing this dilemma for years. So what’s the truth?
DEET and Sunscreen Together:
The debate on application of sunscreen with DEET is an old one.
Using DEET insect repellent with sunscreen decreases the effectiveness of the sunblock by about 30-40%. So if using both at the same time, be prepared to re-apply sunscreen more often than if using sunscreen alone.
There are several commercial products that combine DEET and sunscreen into one cream or lotion. These are generally not as good as using two separate products.
Which one applied first?
For several years, the Travel Medicine community has preached application of sunscreen first and then DEET applied over the sunscreen.
This information comes from the CDC Yellow Book. Put the sun block on first, allow a period of about 5-10 minutes to allow the sunscreen to dry, prior to applying DEET insect repellent.
This new advice, from the International Society of Travel Medicine, says to apply the DEET first and the sunscreen after. This has prompted a good discussion and a look at the evidence for both sides... or lack of evidence.
Evidence, what evidence?
Simply, there is not good evidence for application of sunscreen before or after DEET based insect repellent.
One of the experts in the discussion believes that the order in which the two products are applied has little effect on the decreased efficacy of sunblock. This decreased protective effect of sunblock when used with DEET is unavoidable.
The doctor then went on to hypothesize that the sunscreen applied over the DEET might interfere with the evaporation of the DEET, thus lowering its ability to repel mosquitoes. He is also quick to point out that he has not found decent literature to back this statement.
The other belief is that DEET might be less absorbed by the skin if sunscreen were applied first, protecting the skin. Obviously, the goal is to reduce the skin's absorption of DEET as much as possible. This is also another theory without decent evidence.
Basically, there is no clear, decent evidence that supports these ideas.
Real Life Application
This should raise the issue that a decent study needs to be done, examining this issue.
Lacking decent evidence, Travel Doctors should be cautious about sticking to the old advice of sunscreen before DEET and inform their traveling patients that the data is unclear.
Author: Phil Sylvester
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