The problem with Prussian blue is that it is by no means a categorical sign of cyanide exposure. One factor necessary in its formation is a very high concentration of cyanide. In terms of the difference between amounts measured in the delousing chambers and homicidal gas chambers, critics explain that the exact opposite of what deniers claim is true. Insects have a far higher resistance to cyanide than humans, with concentration levels up to 16,000ppm (parts per million) and an exposure time of more than 20 hours (sometimes as long as 72 hours) being necessary for them to succumb. In contrast, a cyanide concentration of only 300ppm is fatal to humans in a matter of minutes. This difference is one of the reasons behind the concentration disparity. Another exceedingly sensitive factor by which very small deviances could determine whether Prussian blue may form is pH. pH could be affected by the presence of human beings. Also, while the delousing chambers were left intact, the ruins of the crematoria at Birkenau had been exposed to the elements for over forty years by the time Leuchter collected his samples. This would have severely affected his results, because unlike Prussian blue and other iron based cyanides, cyanide salts are highly soluble in water.