There is no need for a gas mask. Gas masks were developed for soldiers who have to remain in a specific area. Biological and radiological agents, which are airborne, are vapors, not gasses. Vapors immediately begin to dissipate once they are released. When you leave an area, the risk of being affected by a vapor diminishes, so you can leave an area and leave the risk behind.
Chemical gasses need to be delivered in large quantities in order to kill or cause injuries. If you smell a vapor or gas, remember to stay calm. If you panic, you have the tendency to breathe faster and therefore will breathe more of the biological, radiological, or chemical agent. Listen to local radio and television broadcasts for information if an airborne attack occurs.