Final answer:
The lowest dose required to develop a visible tan is determined by the amount of UV radiation the body is exposed to. Sunscreen with a sun protection factor (SPF) rating can help block UVB radiation that causes tanning. However, there is a limit to how much radiation our melanin can absorb, so overexposure to the sun can still be dangerous.
Step-by-step explanation:
The lowest dose required to develop a visible tan is determined by the amount of UV radiation the body is exposed to. When the skin is exposed to moderate levels of radiation, the body releases melanin, a pigment that helps block UV penetration and protect deeper skin tissue. Sunscreen with a sun protection factor (SPF) rating can help block UVB radiation that causes tanning. However, there is a limit to how much radiation our melanin can absorb, so overexposure to the sun can still be dangerous.