Pool chemicals are the most dangerous thing most people keep in a garage, and almost every serious incident comes from the same small set of mistakes. None of them are complicated to avoid.
Never mix chlorine products with acid
This is the one that sends people to hospital. Combining a chlorine product with an acid — muriatic acid, dry acid, or even some household cleaners — releases chlorine gas. It happens immediately, it is heavier than air so it pools where you are standing, and a small amount is enough to cause lasting lung damage.
The rule is absolute: chlorine and acid never meet outside the pool. Not in the same bucket, not in the same skimmer, not one straight after the other into the same spot of water.
If you have added one chemical and want to add another, wait for a full turnover of the water — several hours — and test before you do. In a bucket, never.
Never mix two kinds of chlorine
Calcium hypochlorite and trichlor are both chlorine products, and mixing them is a genuine fire hazard rather than a theoretical one. The combination can ignite, and it can do so with nothing but a trace of moisture to start it. Keep them in separate containers, use a separate scoop for each, and never store one above the other.
Chemical into water, never water into chemical
When a concentrated chemical is pre-dissolved, it goes into a bucket of water — not the other way round. Pouring water onto a concentrated acid or a dry oxidiser can boil the surface instantly and throw the chemical back out of the bucket.
Storage
- Cool, dry and out of sunlight. Heat degrades chlorine and pressurises containers.
- Acids and chlorine products on opposite sides of the space, never on the same shelf and never with one above the other.
- Original containers with their labels. A decanted chemical in an unmarked tub is how the wrong two end up mixed.
- Off the floor, where a leak or a flood cannot reach them.
- Locked, if children or animals can get in.
While you are adding anything
- Eye protection and gloves. Splashed acid does not give you time to react.
- Outdoors, upwind, with the pump running so what you add is carried away rather than sitting in one place.
- One chemical at a time, and test before the next one.
- Never swim until the level has come back into range — for chlorine that means below about 4 ppm.
Calculators
Read the label, every time
Products that look alike differ in strength. Liquid chlorine is sold anywhere from 10% to 12.5%, and cal-hypo from 47% to 73%. A calculator can only tell you how much active ingredient your water needs; the percentage on the bottle in your hand decides how much of that bottle that is. Our dosing calculators ask for the product and its strength for exactly this reason.
How long after adding chlorine can we swim?
Test rather than time it. Once free chlorine is back below about 4 ppm and pH is in range, the water is fine. After a full shock that usually takes several hours with the pump running.
Can I store chemicals in the pool shed with the pump?
Only if it is well ventilated and dry. Chlorine vapour corrodes metal, and a pump, a heater and a control board are all metal. Many equipment failures trace back to chemicals stored alongside them.
What do I do if I have already mixed something?
Leave the area immediately and get everyone else out, including upstairs if it is an enclosed space. Do not try to dilute or neutralise it. Ventilate from outside and call your local emergency number if anyone is coughing or short of breath.
Is liquid chlorine safer than tablets?
It is easier to handle and it adds no stabiliser, but it is still a corrosive that will burn skin and destroy clothing. Neither is safe to be careless with.