  The synthesis of vitamin D is strongly linked to the availability of solar
energy. For a long time, it was clear that using the erythemal irradiances is a
good choice to calculate the exposure time. Several authors argue that this
method of minimum exposure time overestimates the calculation of the solar
irradiance of vitamin D. In this paper, the vitamin D and erythemal irradiances
are calculated on the basis of the spectral solar irradiance in Urcuqu\'i
(Ecuador). From these results, we obtain the minimal exposure time. It was
found that there is a difference between the use of the erythemal irradiance
and the vitamin D one; namely, the exposure times were too large (almost a 100%
difference) for the erythemal irradiance.
