Predominantly used physical sunscreen agent, Zinc Oxide, is a broad spectrum sunscreen which protects skin from both UVA and UVB rays (both types). Generally, it helps a lot in preventing tanning. But, there is a loop hole here. The abilities of the sunscreen depends on the strength of its protection. For example, if you are using a spf 15 sunscreen, it filters out 92% (rounded figure) of the sun's rays and still allows 8% to go through. Those 8% of the sun's rays can still harm the skin.
Similarly, when talking about Titanium Dioxide, it filters out all UVB rays but only half of UVA rays which reduces the strength of the sunscreen considerably. In spf 15, 8% of rays are still passing through to the skin and also, UVA rays are separately affecting your skin because the sunscreen can't block them. Also, Titanium Dioxide is responsible for generating free radicals in the body unless it is coated.
The major issue with chemical sunscreen is the stability factor. The chemical ingredients breakdown on exposure to sunlight. Yes, that's a contradiction to the function of the product. That's the main reason why a sunscreen has to be reapplied after every two hours. Also, they have a tendency to generate free radicals in the body which can also trigger cancerous reaction within the body.
Also, let me say here that the "Helioplex" of Neutrogena is a stabilising formula (read this post for more details) to prevent the breakdown of the chemical sunscreen ingredients which makes its sunscreens effective on a longer length of time. But, again the strength of sunscreen decides the amount of rays passing through to the skin and they can cause tanning.
Also, a sunscreen which doesn't have PA ratings gives no protection against UVA rays so still your skin is vulnerable to tanning and aging. PA ratings gives protection against the UVA rays and spf gives the protection strength against the UVB rays so a sunscreen needs to have both these factors mentioned for it to be a broad-spectrum sunscreen.
Recommended Sunscreens:MVO Moisturizing Face Screen
Neutrogena Ultra Sheer Dry-Touch Sunblock
Sunscreen: Role in anti-aging Skin Care
PS: Just checked out that Hermes stores are there in Delhi, Mumbai and Pune. Btw, any one has idea if there is a Hermes counter / Store in the Dubai airport, those duty free ones???