The best way to protect yourself and others from harmful chemicals or smoke in the air is to go indoors immediately. This is called shelter-in-place. Emergency officials may tell you to shelter-in-place during a chemical release instead of evacuating. This will avoid exposure to the high outdoor concentration of a chemical.
Sheltering indoors works because the outdoor air does not mix quickly with the indoor air when the building is closed. The fresh air inside a building proves protection for several hours while the hazardous plume is carried downwind and is diluted by clean air.
To maintain the fresh air inside the building before the plume arrives, it is important to implement shelter-in-place immediately (see how to shelter-in-place below) when instructed. This will minimise exposure to toxic vapours and smoke.
It is necessary to consider that all buildings have small gaps and cracks, and some have air vents and chimneys. Some of the harmful vapours in the outside air will gradually infiltrate through these openings into the shelter. Eventually, sometime after the highest concentration of the hazardous plume has passed the shelter, outside air will be cleaner that the air inside the shelter. Sophisticated software allows emergency responders to calculate this time point. They will then issue instructions to ventilate the shelter and building and go outside or leave the area. For this reason, it is very important to promptly leave the shelter when advised to do so, to avoid continued exposure. Hazardous vapours that might have infiltrated the building will eventually dissipate.