By Tim John, DVM

After reading over your horse’s fly spray bottle, you think:

Cool! This fly spray has sunscreen in it, too! Bonus.

Sometimes, though, it pays to step back and look at the bigger picture. What is the most likely reason that sunscreens were added to these spray formulations? Cancer protection is one of the reasons, yes. But the main reason is that the active ingredients in most fly sprays are destroyed by UV light. Pyrethroids (see What’s in My Horse’s Fly Spray, Part 1) can be very sensitive to UV degradation, so companies added in sunscreens as a way of protecting the effectiveness of the fly spray formula.

So what’s the big deal?

Image credit: Ellen Sinding,
“Summer sun” (via Flickr)

As we know about most things, there is always a tradeoff. Sunscreens protect the skin by penetrating deeper layers to block the damage caused by UV rays. However, they also help other compounds in the same formulation to penetrate to a greater extent.

That’s right: Research has shown that sunscreens can enhance the absorption of pesticides into an animal’s body. I’m not really sure that I want that.

In fact, our friends to the north (Canada) are so unsure of this combination that they do not allow human products with sunscreen and DEET in the same formulation. Kinda makes you wonder. Here in the U.S., the EPA and FDA asked for comments in 2007. We’re still waiting for their decision.

In the meantime, now you have something else to ponder in our never-ending battle against our flying nemeses.