A friend who burns easily is travelling to Africa during the dry season. What is the strongest sunblock on the market, and is it actually sensible to use - that is, would it potentially prevent sweating which may not be healthy for the body?
The strongest sunblock would be to wear a big hat and long sleeves and pants (preferably white clothing). I mean this with all sincerity. The only way to truly block the sun in the most effective way is to take steps to prevent it from reaching your skin.
In addition to sunblock, long sleeves, a hat and sunglasses, you should also consider Zinc creams (Zinc Oxide). It's a physical barrier, resists sweating and for these reasons is what cricketers wear when they have to play in the sun all day. It's also hypoallergenic and is used to prevent some skin conditions (again by providing a physical barrier between the skin and the environment).
My brother burns really easily and has started using SPF 75
He has noticed a difference between this and SPF 30.
When looking at clothing for SPF protection, you can use the guide at the Skin Cancer Foundation. I followed this that last 2 times I was in India.
In Australia back when the ozone layer started to disappear and everybody started wearing strong sunblock they used to tell us that anything with a SPF (sun protection factor) above 30 did not make any difference. They have stopped saying this I think due to commercial pressure. There is more on this in the Wikipedia article saying upper limits for claimed SPF is enforced in several countries.