It is only when one uses an exposure that is not applicable to the human situation that one sees the tumors. It should also be noted that instilling the slurry of fibers into the lung is a traumatic experience for the animals and the process may be irritating, making it easier for the fibers to attack the cells of the lung. And, while the upper respiratory tract is quite efficient in trapping lung fibers so they do not deposit in the lung, instilling the fibers bypasses this major defense mechanism of the respiratory tract. On the other hand, because the structural differences between rats and humans, the quantity of fibers inhaled by rats is smaller than the corresponding quantity that could be inhaled by humans. To get the larger number of fibers per kg body weight into the lung of a rat, one has to invoke a non-physiological method of exposure, i.e., installation.
Installation is justified in reaching an exposure level that is possible in man.
Fibers that are one-eighth inch or longer are not likely to get past the upper respiratory tract and therefore cannot deposit in the lung. Even blown-in insulation usually consists of clumps of long fibers. It is unlikely that the average homeowner who installs or replaces insulation in his home will have a significant exposure to man-made mineral fibers.