When the emergency is over
As soon as it is safe, authorities will tell you it is okay to leave your building. At that time, they will tell you to open all your doors and windows and turn on all those fans you turned off earlier so the house or building will air out.
Some of the chemical may have seeped into your home. Though it may not reach toxic levels in the short term, it may be enough to cause harm over the long term. Local officials know what the chemicals can do and will give you detailed instructions.