Answer from Tom Baker:
This is a common call we receive, mainly involving swimming pools. Nipac is a mixture of acids and reacts with bleach to release chlorine gas. Dealing with this situation is quite simple: firstly you should evacuate the area to minimize exposure. If safe to do so the area should be ventilated to allow the gas to escape. Chlorine is heavier than air and may accumulate in low lying areas but can be knocked down with water. Once the reaction is complete, appropriately equipped personnel with respiratory protection can go in and clean up the spill.
Tom Baker is a senior responder at the National Chemical Emergency Centre