Final answer:
SPF 20 sunblock provides protection for approximately 20 times longer than without protection, but it should be reapplied every two hours. Broad-spectrum protection against both UVA and UVB rays is important for preventing skin damage.
Step-by-step explanation:
The Sun Protection Factor (SPF) of a sunscreen or sunblock indicates its effectiveness in blocking UVB rays. An SPF 20 sunblock would typically provide enough protection to prevent sunburn for 20 times longer than without any sunblock. The effectiveness of sunblock depends on various factors, including skin type, intensity of sunlight, and whether the product is water-resistant or not. However, it is recommended by health organizations to reapply sunscreen every two hours to maintain optimal protection, regardless of the SPF rating. It is important to remember that no sunblock is waterproof, but some products may be water-resistant, which can be more effective when sweating or swimming.
It is also important to select a sunscreen that provides broad-spectrum protection, meaning it shields from both UVA and UVB radiation. While the SPF value indicates the level of protection from UVB rays, ingredients such as titanium dioxide, zinc oxide, and avobenzone are compounds often found in sunscreens that help protect against UVA rays. This broad-spectrum protection is key to preventing skin damage and potential long-term effects such as skin cancer.