The UV index is divided into categories to indicate risk levels: low (0-2), moderate (3-5), high (6-7), very high (8-10), and extreme (11+). Low levels suggest minimal risk, while extreme levels require extra precautions. For example, at high or very high levels, UV rays can cause skin damage in as little as 15 minutes. Understanding these categories allows you to adjust your outdoor plans and take appropriate protective measures.