Pool chemical safety

The rules that prevent most injuries (CDC data)

Pool chemicals send thousands of people to U.S. emergency rooms every year. A CDC report covering 2008–2017 estimated a median of 4,535 emergency department visits a year. In the detailed 2015–2017 data:

• at least 56% of injuries happened at a residence, not a public pool;
• about 36% of patients were under 18;
• the most common diagnosis was poisoning (39%), mostly from breathing fumes rather than swallowing, followed by skin or eye irritation (28%) and chemical burns (19%);
• about two thirds happened between Memorial Day and Labor Day.

The cases behind those numbers show three repeated patterns: breathing fumes or dust when opening a container, chemicals left where children could reach them, and chemicals added to the water right before someone got in. Each of the rules below targets one of these.

1. Read the whole product label, or the safety data sheet (SDS), before you use a product for the first time.
2. Open containers outdoors, upwind, at arm's length, wearing goggles and gloves.
3. Use one product at a time, with its own clean scoop or jug.
4. Add chemicals to water, never water to chemicals.
5. Keep everyone out of the pool while you add chemicals, and until levels are back in range (at least 30 minutes after acid).
6. Lock chemicals away from children and pets, with chlorine and acid stored apart.

Never mix

Some of the most dangerous home accidents with pool chemicals happen when two products meet in concentrated form: in a bucket, a scoop, a skimmer, a floater, or on a shelf. The CDC's wording is plain: never mix chlorine products with acid, because this could create toxic gases.

Combinations to avoid, and what happens

Never combine	What can happen
Any chlorine (liquid chlorine, bleach, cal-hypo (calcium hypochlorite), dichlor, trichlor) + acid (muriatic, dry acid)	Chlorine gas (CDC)
Two different chlorine products, for example cal-hypo + trichlor tablets, or cal-hypo + dichlor	Heat, toxic gas, and in the worst case fire or explosion; the CDC storage poster says not to mix different types of chlorine products
Cal-hypo + moisture with oil, fertilizer, leaves, paper, or other combustibles	Decomposition that releases oxygen and chlorine and can start a fire that is hard to put out (OSHA)
Chlorine or bleach + ammonia-based cleaners	Chloramine gas (Poison Control)

In practice: don't pour shock into a skimmer that holds trichlor tablets; don't put a new type of tablet into a floater or feeder that held another type; don't reuse a bucket or scoop across products; and keep 30 minutes and some distance between chlorine and acid when you dose. More on the differences between chlorine types in pool chlorine types, and on dosing order in what order to add pool chemicals.

Storage

From the CDC's pool chemical storage poster, adapted for a home shed or garage:

• Follow the storage directions on each label.
• Keep incompatible chemicals apart, especially acid and chlorine: different shelves, ideally different sides of the room.
• Lock them up to protect children and animals.
• Keep them dry, and never mix different chemicals, including different types of chlorine.
• Keep them cool, ventilated, and out of direct sunlight. Heat also weakens liquid chlorine: PHTA data show 12.5% sodium hypochlorite loses half its strength in about 48 days at 95 °F, versus about 180 days at 77 °F.
• Keep products closed in their original, labeled containers.
• Store liquids low, below solids, so a leak can't drip onto something else.
• Never reuse empty containers. Follow the label for disposal, and ask your local hazardous waste program about anything unlabeled.

Also keep pool chemicals away from gasoline, lawn fertilizer, paint thinner, and the lawnmower, which are often stored nearby in a garage.

Adding chemicals

• Test first, and measure the dose instead of estimating. Our calculators give the amount for your product and its strength; the muriatic acid calculator also splits large acid doses into steps.
• With the pump running, pour slowly and close to the water surface in front of a return jet, so the product spreads before it can settle.
• If the label says to pre-dissolve a product, do it in a clean bucket of pool water: add the chemical to the water. Calcium chloride gets hot when it dissolves.
• Don't add chemicals when anyone is in the pool, and don't let people in right after. Wait for the time on the label and until levels are back in range.
• Rinse measuring tools with plenty of water after each product, away from the chemical storage area.

Protective gear

The CDC recommends safety goggles, gloves, and a mask when handling pool chemicals. For a homeowner, that means:

• splash-proof goggles (regular glasses leave gaps at the sides);
• chemical-resistant gloves, such as nitrile;
• closed shoes, and long sleeves when pouring acid or large amounts of chlorine;
• a mask or respirator as the product label directs, especially for dusty granular products.

If something goes wrong

Poison Control

Poison Control in the U.S. is 1-800-222-1222. The line and the online tool webPOISONCONTROL are free, confidential, and available 24/7, and the call is answered by experts who can tell you whether home care is enough. Save the number in your phone and write it on the inside of your chemical cabinet door. Call 911 instead if someone collapses, has a seizure, or has trouble breathing.

This page is general safety information, not medical advice. Your product labels and SDS come first. How we source our numbers and limits is on the methodology page; our full disclaimer is in the terms. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.

FAQ

Can you mix cal hypo and dichlor?

No. Never mix different types of chlorine with each other, in a bucket, a skimmer, or a feeder. The CDC's storage guidance says not to mix different types of chlorine products, and cal-hypo in particular can react violently with other chemicals. If you use both, add each directly to the pool at a different time, and store them apart.

Can pool chlorine burn your skin?

Concentrated products can. Chemical burns were about 19% of pool chemical injuries seen in U.S. emergency rooms in 2015–2017 (CDC). Normal pool water at recommended levels is not in that category, but it can irritate skin and eyes. If concentrated chlorine touches your skin, remove wet clothing, rinse with plenty of water, and call Poison Control.

How should pool chemicals be stored?

In their original closed containers, in a cool, dry, ventilated, locked place out of direct sun, with chlorine and acid kept apart and liquids stored below solids. Keep them away from fuel, fertilizer, and other combustibles.

Sources

• CDC MMWR, Pool chemical injuries in public and residential settings, United States, 2008–2017, and New York, 2018: cdc.gov
• CDC, Store pool chemicals safely (poster): cdc.gov (PDF)
• OSHA, Hazard information bulletin: calcium hypochlorite: osha.gov
• Poison Control, Chlorine gas: get the facts: poison.org
• Poison Control, Splashed a poison in your eye?: poison.org
• Poison Control, Contact us (phone and webPOISONCONTROL): poison.org
• PHTA fact sheet, Sodium hypochlorite (strength loss by temperature): phta.org