Tan is caused mainly by UVA and UVB rays in sunlight (partially by visible light too). If tan to be prevented entirely, avoidance of sun exposure and use of physical barriers like umbrella and hats are advised.

Sunscreens are products that we can apply on the skin which prevent sunrays from entering the skin. Physical sunscreens block light physically by forming a layer while chemical sunscreens absorb the light and do not let it enter the skin. Nowadays, all good sunscreens have both physical and chemical components.

Protection from UVB is measured by SPF (sun protection factor), and a rating of SPF 30 or above is suffucient for protection from daily exposure. UVA protection is checked by PA rating, and PA+++ is considered to be good protection. If you are looking for an ideal sunscreen, it should have broad spectrum action with both SPF and PA rating, containing physical components like titanium dioxide, zinc oxide and chemical components like oxybenzone, avobenzone.

The sunscreen should suit your skin type e.g. non comedogenic sunscreens for acne prone screen and lotion based ones for dry skin. Sunscreens need to be applied every 2-3hourly for best effect.

Visit a dermatologist to know your skin type and what sunscreen would be best for you based on your age, degree of sun exposure and need of protection.