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