You should ensure your sunscreen will protect you from both UVA and UVB radiation. This means using a broad-spectrum sunscreen. No matter what the stated SPF rating, no sunscreen provides 100% protection against UV radiation and some UV radiation will always reach the skin. Using SPF30+ sunscreen means you are protected from about 96.7% of ultraviolet radiation. The amount of UV radiation transmitted to the skin increases as the length of exposure time increases.

Source:
Giles P, Homes G. The effective use of sunscreens. Cancer Update in Practice. The Cancer Society of New Zealand January 2003.