A biological or chemical attack is always a threat, whether foreign or domestic.

Shortly after the Sept. 11, 2001, Al Queda announced that it was attempting to obtain biological and chemical weapons for another potential attack.

The severity of an attack depends on a number of different factors, according to the Department of Homeland Security.

     Many variables affect the concentration of a chemical including wind and the volatility of the chemical. The release of toxic chemicals in closed spaces (e.g., in subways, airports and financial centers) could deliver doses high enough to injure or kill a large number of people. In an open area, a toxic chemical cloud (plume) would become less concentrated as it spreads and would have to be released in a large quantities to produce a lot of casualties.

WikiHow recently posted a list of 23 steps to take during a chemical attack.

Check out the above photo gallery and use the necessary steps in the event of a chemical attack, courtesy of WikiHow.