A few measurements researchers use to quantify sun exposure are in units of “minimal erythemal doses” (MEDs) or units of “standard erythema doses” (SEDs). The names come from the concept that human skin generates an erythema (redness) in response to the sun’s intensity over a length of time.
When the MED unit was developed, they wanted one unit of MED to produce a slight observable redness of the skin, hence the quite literal name “minimal erythemal dose.” The problem, as we all know, is that one MED is going to be different in different skin types. A type III skinned person (somewhat hard to burn) is going to require more sun intensity, more time than a type I skinned person to turn pink.
Thus an MED isn’t really a standard measure; it is variable based on skin sensitivity to UV exposure. And it doesn’t consistently define UV intensity. Nevertheless, different agencies assigned different values to the MED in units of J/m² (Joules per square meter), and MED is still used in the tanning industry today. For further confusion, sometimes they adjust MED for skin type.
About 10 years ago, the International Commission on Illumination wanted to introduce a more standardized value to sun intensity and its effect on skin. So they proposed the SED, which is one value (100 J/m²), regardless of skin type. Depending on whose standards you go by or whether or not you adjust for skin type, 2 SEDs usually equals 1 MED, but not always. The important thing here to understand is that the SED is standardized and always equals 100 J/m², while the MED could have a different definition depending on the context and the way the scientist uses it in the paper.
For skin type I, 1.5 SED will produce a slight observable erythema. In skin type II, an SED of 2 will produce a slight observable erythema. And in skin type III, an SED of 3 will produce a slight observable erythema.
Since SED takes into account both intensity and time, a dose of 1 SED will take longer to achieve on a cloudy day than a clear day. Also, 1 SED will be delivered faster at higher elevation and near the equator, where the sun is more intense on a clear day.
During summer, a full ambient day in the tropics provides about 70 SED, 60 SED in southern Europe and 45 SED in the UK (and respective latitudes). However, it is unlikely that anyone would lie in the unshaded sun for an entire day, and so it is unlikely that anyone would receive more than 20 SED to any one part of the body in a single day (note that 20 SED would result in an intense sun burn without the use of sunscreen).
This is useful for vitamin D research, to an extent, because you can measure vitamin D production in response to a quantifiable amount of UV exposure. The challenge, however, is that the UV spectrum that influences erythemal response is broader than the spectrum that induces vitamin D production. Depending on the frequency of the wave, your skin could burn without any vitamin D production.
To further complicate the issue, depending on where you live, the ratio of the amount of rays that influence vitamin D production to the amount of rays that influence erythemal response can be different. Studies show that the higher the latitude, no matter the season, the lower the ratio of vitamin D rays to erythemal rays compared to latitudes closer to the equator. In other words, if the same person were exposed to 3 SEDs on the equator and 3 SEDs in New York, they would produce a little more vitamin D on the equator than in New York, despite getting the same amount of SEDs.
This is also true based on the time of day. Studies show that the higher ratio of vitamin D rays to erythemal rays occurs around noon, and times before and after will have lower ratios. In other words, if the same person attempted to get 3 SEDs starting around noon, they will produce more vitamin D than if they attempted to get 3 SEDs starting at 2pm.
Pope SJ, Holick MF, Mackin S, Godar DE. Action spectrum conversion factors that change erythemally weighted to previtamin D3-weighted UV doses. Photochem Photobiol. 2008 Sep-Oct;84(5):1277-83. Epub 2008 May 29.
As you can see, vitamin D and sun exposure can get complicated quickly. Right now, recommendations regarding sun exposure take into account erythemal response and measurements like MED, SED and Ultraviolet Index (UVI). Moving forward, hopefully vitamin D producing rays will also factor into that equation, and hopefully there is a simple way to communicate that kind of information to the public.
In summary, the key points are: