Sunscreen has usage has increased significantly the past 3-4 decade while skin cancer rates have also gone up. If sunscreen is supposed to stop skin cancer why is this?

The short version: sunscreen does reduce some skin cancers, but the rising rates are real and the story is more complicated than "sunscreen failed."

The evidence for sunscreen benefit is real but uneven. A randomized trial in Australia found daily SPF 16 use cut invasive melanoma by about 73% and squamous cell carcinoma by about 39% [4]. A large Norwegian cohort found an 18% melanoma reduction with SPF 15 or higher [4]. But the same review shows basal cell carcinoma barely moved, and a meta-analysis of observational studies found no melanoma reduction at all [12]. The strongest evidence comes from one Australian trial, and the observational data is genuinely mixed.

The rising incidence has a simpler explanation than sunscreen failure. Melanoma rates were already climbing steeply before sunscreen became common, from about 1 in 100,000 in the 1930s to 6 in 100,000 by 1980 [14]. Sun exposure patterns changed dramatically: more intermittent, intense sunbathing and vacations in sunny places, which is the pattern most strongly linked to melanoma [14]. People also use sunscreen to stay out longer, and many apply far too little, too late, and never reapply [9]. Sunscreen is a filter, not a force field.

The uncomfortable truth is that the best evidence for sunscreen comes largely from one well-run trial, and the observational literature is conflicted. The industry-funded studies and the public health messaging rarely show you this split. What nobody has adequately studied is whether modern sunscreen ingredients themselves, absorbed through skin into blood, have long-term effects. That data does not exist.

My call: sunscreen clearly prevents some skin cancers, especially squamous cell and melanoma, but the rising rates reflect changing sun behavior and detection, not sunscreen failure. Confidence: moderate.