What time do we need to sunbathe and what time does not?
Sunlight produces infrared (heat), visible light (colors), ultraviolet that cause sunburn and cellular changes that predispose to cancer. This radiation is highest between 10 am and 4 pm. At these times the level of UV radiation is bigger, more aggressive and carcinogenic.

How long can we be a day in the sun without risk? What has the ability to get our brown skin?
The tan is a defense mechanism of the skin, because it serves a purpose to filter UV rays. Sun exposure in normal skin should be done by increasing exposure of 10 minutes each day, from the first that does not exceed 15 minutes, avoiding the midday hours. Within 10 days the skin will be ready to withstand the sun without any problems except for accumulate of sunlight (important for preventing skin cancer).

It must be borne in mind that each skin type is one, the blond and red hair has very little ability to tolerate sunlight, the chestnut has a tolerance that can be considered normal, and dark-skinned tolerate higher speed in decision-sun .

You always need to use sunscreens? What protection factor? How often we apply them to us?
The only factor Sunscreen offer an increase in the time of tolerance to sunlight (particularly by UVB rays), but not canceled, in general, the effects of the sun in its entirety.
Will prove useless in spectra of action and to filter out UVA rays and visible light (except Oats 60). In these cases can be effective filters based on titanium dioxide.

Protective factors should be applied 30-45 minutes before the exposure to the sun, being removed by sweat and water. The rates of protection are the factor by which to multiply for us to know how long it stands for, example, a protection factor of 20 allows 20 minutes exposure to the sun as if it were just 1 minute. The filter 50 allows 50 minutes as if it were 1. We may find filters from factor 2 to factor 50 or more that are called total or screens.