I have tried to find a direct answer to this, but it's very difficult as all results direct me to cancer conversations. Suppose that one is dealing with extreme temperatures and sun exposure; on a clear day swelling to high temperatures, what's more immediately dangerous - heat or uv? Is it safer in the shade if it's hot enough that the ambient temperature remains the same?
Heat stroke can be deadly and it can occur in a relatively short period of time, even under an hour.
When heat stroke occurs, the body temperature can rise to 106°F or higher within 10 to 15 minutes. Heat stroke can cause permanent disability or death if the person does not receive emergency treatment.
UV from sun exposure can be deadly, but only after years of exposure and/or multiple severe sunburns, and then only indirectly via the secondary cause of cancer.
Most skin cancers are a result of exposure to the UV rays in sunlight. Both basal cell and squamous cell cancers (the most common types of skin cancer) tend to be found on sun-exposed parts of the body, and their occurrence is typically related to lifetime sun exposure.
In short, no one dies of UV exposure from the sun in an afternoon, but people do die in an afternoon due to heat, making it easily more immediately deadly.