October 3, 2026

Sunburn can happen even after applying sunscreen, especially if you use too little, skip reapplication, or expose the product to excessive heat. Does that mean your sunscreen has failed? Most of the time, sunscreen “fails” for simple reasons: too little was applied, it was not reapplied, or the product was stored incorrectly or has passed its expiry date. A quick sunscreen test at home can help you understand whether a formula is absorbing UV or whether you have applied it evenly. However, no home test can tell you the exact SPF. That requires laboratory testing. If you have a half-used bottle from last summer and are wondering whether it is still usable, checking its expiry date, appearance, and storage history is more useful than relying on a DIY SPF test. In this guide, we explain four ways to check your sunscreen at home, what each test can and cannot tell you, and how to get more reliable protection from your daily SPF 50 sunscreen . What Is SPF and Why Can Sunscreen Lose Its Effectiveness? SPF, or Sun Protection Factor, indicates how much UVB exposure is required to cause sunburn on protected skin compared with unprotected skin. As a general comparison: SPF 30 filters approximately 97% of UVB. SPF 50 filters approximately 98% of UVB. SPF 70 filters approximately 98.6% of UVB. The difference between SPF 30 and SPF 50 may look small numerically, but SPF 30 allows roughly 50% more UVB through than SPF 50. For a deeper comparison, read our guide to SPF 30 vs SPF 50 vs SPF 7…

October 1, 2026

UVA and UVB are two types of ultraviolet radiation from the sun that affect the skin differently. UVA penetrates deeper into the skin and contributes to tanning, pigmentation, premature ageing, and skin cancer. UVB primarily affects the skin's surface, causing sunburn and DNA damage. SPF measures UVB protection, while PA indicates UVA protection , so choose a broad-spectrum sunscreen that protects against both. Key Takeaways UVA: penetrates deeper and contributes to tanning, pigmentation, ageing, and skin cancer. UVB: primarily affects the skin's surface and causes sunburn and DNA damage. SPF: measures UVB protection. PA: indicates UVA protection. Broad spectrum: protects against both UVA and UVB. Daily sunscreen: choose SPF 30 or higher with UVA protection. Long outdoor exposure: SPF 50 with broad-spectrum protection can provide a higher margin of UVB protection. Difference Between UVA and UVB UVA and UVB radiation both come from the sun, but they affect your skin differently. UVA rays account for about 95% of the UV that reaches Earth. They penetrate into the dermis, contribute to tanning, pigmentation, wrinkles, and skin cancer, and remain relatively consistent throughout daylight hours and across the year. UVA can also pass through clouds and window glass. UVB rays account for about 5% of UV reaching Earth. They are high-energy rays that primarily affect the skin's surface, causing sunburn and DNA damage. UVB is strongest around midday and varies with season and atmospher…