The International Society of Travel Medicine has a really wonderful list-serve that discusses topics on travel health. There is a great exchange on a new article about sunscreen selection and use…specifically using sunscreen and DEET insect based repellents. The article has a lot of other good information and is presented very well, but the issue being focused on is the DEET and sunscreen advice. Read the article here: http://www.ewg.org/2010sunscreen/EWG_sunscreen.pdf
DEET and Sunscreen Together:
The debate on application of sunscreen with DEET is an old one. Everybody should now know that using DEET insect repellent with sunscreen decreases the effectiveness of the sunblock by about 30-40%. Thus, 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 . A period of about 5-10 minutes should be given to allow the sunscreen to dry, prior to DEET insect repellent application. This new article (mentioned and linked above) states 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.
Simply, there is not good evidence for application of sunscreen first 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 not 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.