Unit of measure, abbreviated nm. One billionth of a meter. The average diameter of a human hair is about 80,000 nm. The influenza A virus, in its spherical form, is about 120 nm, intermediate in size for a virus. Thus its size is between that of a 20 nm rhinovirus (a cold virus) and a 200 nm Ebola virus, as seen here. Note that this is more than 100 times smaller than a human red blood cell or ragweed pollen.

experts have warned that face masks could prove useless in combating the killer virus. Scientists have warned that masks would need to be changed at least twice a day to make them in any way effective - meaning tens of millions would be needed. And they could cause a public health hazard in themselves if they are not disposed of properly, by actually helping the flu spread. So, does wearing a face mask does really protect you and what would be the best mask to buy?

A chemical threat needs a different approach, because chemicals come as mists or vapors that are largely immune to particulate filtration. The most common approach with any organic chemical (whether it be paint fumes or a nerve toxin like Sarin) is activated charcoal.

Charcoal is carbon. (See this Question of the Day for details on how charcoal is made.) Activated charcoal is charcoal that has been treated with oxygen to open up millions of tiny pores between the carbon atoms. According to Encyclopedia Britannica:

The use of special manufacturing techniques results in highly porous charcoals that have surface areas of 300-2,000 square metres per gram. These so-called active, or activated, charcoals are widely used to adsorb odorous or coloured substances from gases or liquids.

The word adsorb is important here. When a material adsorbs something, it attaches to it by chemical attraction. The huge surface area of activated charcoal gives it countless bonding sites. When certain chemicals pass next to the carbon surface, they attach to the surface and are trapped.