Water & Air / Compounds / Calcium hypochlorite (Cal-hypo / HTH pool shock)

Calcium hypochlorite (Cal-hypo / HTH pool shock) in water and air: safety profile

High risk

Not medical or professional safety advice, and not a substitute for a qualified clinician — consult one. Full disclaimer →

Caustic ingestion produces esophageal corrosion plus chlorine-gas evolution in stomach acid; pediatric hypochlorite-poisoning is a documented poison-control consult class. ASPCA APCC documents companion-animal pool-chemical ingestion as a frequent emergency-vet presentation.

What is calcium hypochlorite (cal-hypo / hth pool shock)?

The IUPAC name is calcium dihypochlorite.

Also known as: CALCIUM HYPOCHLORITE, 7778-54-3, Calcium hypochloride, Hypochlorous acid, calcium salt.

IUPAC name
calcium dihypochlorite
CAS number
7778-54-3
Molecular formula
Ca(ClO)2
Molecular weight
142.98 g/mol
SMILES
[O-]Cl.[O-]Cl.[Ca+2]
PubChem CID
24504

Risk for people

High risk

Caustic ingestion produces esophageal corrosion plus chlorine-gas evolution in stomach acid; pediatric hypochlorite-poisoning is a documented poison-control consult class. ASPCA APCC documents companion-animal pool-chemical ingestion as a frequent emergency-vet presentation.

Regulatory consensus

4 regulatory and scientific bodies have classified Calcium hypochlorite (Cal-hypo / HTH pool shock). The classifications differ — that's the data.

AgencyYearClassificationNotes
EPA—Registered antimicrobial pesticide under FIFRA. NSF/ANSI 60 certified
DOT—UN1748, Calcium hypochlorite, 5.1 (corrosive), PG II
NFPA—NFPA 400 Chapter 17 — oxidizer storage requirements. Separate from organics and other pool chemicals
OSHA—No PEL established for cal-hypo. Chlorine PEL applies for evolved gas

Regulators apply different standards of evidence — animal-data weighting, exposure-pattern assumptions, epidemiological power thresholds — which is why two scientific bodies can review the same data and reach different conclusions. The disagreement is the data.

Where you encounter calcium hypochlorite (cal-hypo / hth pool shock)

  • Swimming Pools — Granular pool shock (HTH, DryTec), Cal-hypo tablets for commercial pools, Superchlorination treatment
  • Drinking Water — Municipal water treatment, Rural/well water chlorination, Emergency water disinfection
  • Wastewater — Wastewater effluent disinfection

Safer alternatives

Lower-risk approaches that achieve a similar outcome to Calcium hypochlorite (Cal-hypo / HTH pool shock):

  • Sodium hypochlorite (liquid bleach)
    Trade-offs: Alternative approach; specific tradeoffs depend on application context, scale, and regulatory requirements. Full hazard assessment of alternative recommended before adoption to avoid regrettable substitution.
    Relative cost: 1.2-2×
  • Salt chlorine generator
    Trade-offs: Removes 95-99% of dissolved contaminants including metals, PFAS, nitrates; wastes 2-4 gallons per gallon produced (improving with newer systems); removes beneficial minerals; $0.05-0.25/gallon; requires pre-treatment for longevity.
    Relative cost: 1.2-2×

Frequently asked questions

Is calcium hypochlorite (cal-hypo / hth pool shock) safe for you?

Caustic ingestion produces esophageal corrosion plus chlorine-gas evolution in stomach acid; pediatric hypochlorite-poisoning is a documented poison-control consult class. ASPCA APCC documents companion-animal pool-chemical ingestion as a frequent emergency-vet presentation.

What products contain calcium hypochlorite (cal-hypo / hth pool shock)?

Calcium hypochlorite (Cal-hypo / HTH pool shock) appears in: Granular pool shock (HTH, DryTec) (Swimming pools); Cal-hypo tablets for commercial pools (Swimming pools); Municipal water treatment (Drinking water); Rural/well water chlorination (Drinking water); Wastewater effluent disinfection (Wastewater).

Why do regulators disagree about calcium hypochlorite (cal-hypo / hth pool shock)?

Calcium hypochlorite (Cal-hypo / HTH pool shock) has been classified by 4 agencies including EPA, DOT, NFPA, OSHA, with differing conclusions. Regulators apply different standards of evidence (animal data weighting, exposure-pattern assumptions, epidemiological power thresholds), which is why two scientific bodies can review the same data and reach different conclusions. See the regulatory consensus table on this page for the full picture.

See Calcium hypochlorite (Cal-hypo / HTH pool shock) in the water app

Look up products containing calcium hypochlorite (cal-hypo / hth pool shock), compare to alternatives, and explore the full data record.

Open in water View raw API data

Sources (5)
  1. EPA Antimicrobial Reregistration — Calcium hypochlorite (sanitizer/disinfectant) (2008) — regulatory
  2. OSHA PEL — Chlorine (29 CFR 1910.1000 Table Z-1) (2020) — regulatory
  3. ATSDR Toxicological Profile for Chlorine (2010) — regulatory
  4. CDC Pool Chemical Injuries — annual ED-visit surveillance (2018) — regulatory
  5. ASPCA APCC — Pool sanitiser pet exposure (calcium hypochlorite, chlorine evolution) (2024) — veterinary

Reference data, not professional advice. Aggregates publicly available regulatory and scientific data; not a substitute for medical, legal, or regulatory advice. Why we built ALETHEIA →