Effect of the variable background emission to the Herschel-derived temperatures and column densities. The panels show results of a Monte Carlo simulation, in which normally-distributed random variations were imposed on a large sample model SEDs. Left: the output temperature for three values of input temperatures, {15,20,25} K. The repetitions of the simulation are shown with light-grey circles. The red boxes indicate inter-quartile ranges, with a horizontal line at the mean value, and the whiskers show ranges that extend to 1.5 times the inter-quartile range. Right: the same for the ratio of input to output column densities as a function of input column density. The blue dashed line shows the standard deviation, which is approximately 30% irrespective of the original column density.