The diffuse (Dif) component of ultraviolet radiation (UVR) plays an important role in the daily exposure of humans to solar radiation. This study proposes a semi-empirical method to obtain the Dif component of the erythemal dose rate, or the erythemally weighted irradiance, (EDRDif) calculated from synchronized measurements of the Dif component of UVR (UVDif) and the global (G) irradiances of both UVR (UVG) and the erythemal dose rate (EDRG). Since the study was conducted in the tropics, results involve a wide range of solar zenith angles to which EDRDif is seasonally dependent. Clouds are the main atmospheric agent affecting Dif radiation. The ratio between Dif and G (Dif/G) showed a quadratic dependence on cloud cover with a coefficient of determination r(2) = 0.79. The maxima of EDRDif were mainly above the moderate range (>137.5 mW m(-2)) of the UV-Index and reached the extreme range (>262.5 mW m(-2)) for the spring-summer period. The fraction of the global daily erythemal dose (daily EDG) corresponding to Dif radiation (daily EDDif) ranged from 936 J m(-2) to 5053 J m(-2) and averaged 2673 J m(-2). Daily EDDif corresponded to at least 48% of daily EDG for a practically cloudless sky. Therefore, Dif radiation is a real threat. Lighter skin people (types I and II) can get sunburnt in a couple of minutes under such an incidence of radiation. Moreover, accumulative harm can affect all skin types.