Burning time is the maximum time to stay unprotected without receiving sunburn. This is calculated from the UV energy equivalent of 1 MED as the basis.

MED: Minimal Erythemal Dose - Is the effective UV dose that causes a perceptible minimum reddening/tanning of previously unexposed human skin. The exact energy equivalent differs by a factor of 2 depending upon individual sunburn sensitivity.

Each value given in each of the categories represents an approximate mid-point value for a range. In the monthly solar reports the sensitive group appropriate to Category 1 is used with a MED of 200 J m-2 effective.

What Skin Type Are You?
Category	People covered		Approximate MED
Category 1: (burners)	most UK indigenous groups		< 300 J m-2 effective
Category 2: (tanners)	mostly non-UK indigenous and non-naturally pigmented groups		600 J m-2 effective
Category 3: (naturally brown)	mostly Asian and middle and south American groups		800 J m-2 effective
Category 4: (naturally black)	African and Caribbean groups		1000 J m-2 effective

Source Health Protection Agency
See also global solar UV index at WHO