A sunscreen that blocks UVB, harm marine life [1] and can cause allergic airways inflammation [2] when breathed (inevitable when it dries on your skin) is hardly the best sunscreen in my option.
My personal choice for to vitamin D synthesis is Avobenzone stabilized with ubiquinone.
The issues I enumerated affects both nano and non-nano sunscreen with different degrees.
The most commons concerns specific of the nano version are the skin absorption and free radical creation when expose to UV radiation -- not exclusive of the mineral sunscreen though, azobenzene is also highly unstable.
We often hear about studies done with the nano crystal because are worst. Experiments done with them are more likely to produce a negative result. But this doesn't mean the "macro" version isn't affected as well.
The aspiration concern is very serious one. Some countries bans spray and power mineral sunscreen of any kind.
Just to be clear. I'm not advocating against using them. Every sunscreen has its place, saying one option is better than other is reductive.
For instance, think an Avobenzone + antiox only chemical sunscreen is a good option to minimize the UVA damage and enable the body to produce vitamin D minimizing the damage created by UVA radiation. But they wouldn't be my choice when going to the beach.
Similarly, Mineral sunscreens don't degrade in the sun and block a wide range of UV radiation. But they are bad for marine wild life. They are a good choice for daily facial sunscreen, specially for skin sensitized by "anti aging" treatments.
Again. There is no such thing as the best sunscreen. Each is best in different use cases.
My personal choice for to vitamin D synthesis is Avobenzone stabilized with ubiquinone.
[1] https://oceanservice.noaa.gov/news/sunscreen-corals.html [2] https://www.sciencedirect.com/science/article/pii/S0041008X1...