11.1 CDC Healthy Swimming and the Model Aquatic Health Code

11.1 CDC Healthy Swimming and the Model Aquatic Health Code

Quick Answer: Cryptosporidium (Crypto) is the leading confirmed parasite in CDC treated-recreational-water outbreaks. Formed stool uses a Giardia clock of about 2 ppm free chlorine at pH 7.5 or less for roughly 25–30 minutes. Diarrhea is a Crypto event: unstabilized water needs a CT of 15,300, classically 20 ppm for 12.75 hours. Cyanuric acid (CYA) makes that clock longer and different — use the current CDC CYA protocol or drain / secondary treatment, not a homemade number. The 2024 Model Aquatic Health Code (MAHC), 5th edition, is a voluntary model. Your authority having jurisdiction (AHJ) may adopt none, some, or all of it.

Chapter 9 taught the first minute on the deck: close the venue (including co-circulating water), classify formed versus diarrhea, net the material without vacuuming, and start a contamination log. This chapter owns the Centers for Disease Control and Prevention (CDC) science behind those clocks, the MAHC as a model — not a hidden federal statute — and the operator systems that make a Saturday diarrhea event survivable. Independent OpenExamPrep teaching uses public CDC Healthy Swimming pages, the 2024 MAHC code and annex, and CDC outbreak reports. OpenExamPrep is not a CDC, CMAHC, or NRPA partner and does not speak for your health department.

CDC Healthy Swimming is surveillance plus hygiene

CDC's Healthy Swimming program exists because treated water still makes people sick. Health departments report outbreaks through the National Outbreak Reporting System (NORS). CDC then publishes summaries that tell operators which germs, which venues, and which seasons keep repeating. That is recreational water illness (RWI) surveillance: not a rumor mill, and not a substitute for your AHJ's closure order.

Crypto is the treated-water outbreak parasite to respect

For 2015–2019, public health officials in 36 states and the District of Columbia reported 208 outbreaks associated with treated recreational water. Almost all (199; 96%) were tied to public (non-backyard) pools, hot tubs, or water playgrounds. Those outbreaks produced at least 3,646 illnesses, 286 hospitalizations, and 13 deaths. Among the 155 outbreaks with a confirmed infectious etiology:

• Cryptosporidium caused 76 outbreaks (49%) and 2,492 of 2,953 confirmed cases (84%).
• Legionella caused 65 outbreaks and all 13 deaths, often in hotel or resort hot tubs.
• 107 of the 208 outbreaks started in June–August, driven by Crypto in pools and water playgrounds.

Crypto oocysts tolerate ordinary free chlorine for days. CDC has reported that at about 1 ppm free chlorine, pH in the usual band, and about 77°F, oocysts can survive more than a week. A pool that tested legal at 9 a.m. can still host an outbreak by the afternoon if a diarrheal accident — or a child who should have stayed home — loaded the water. That is why Healthy Swimming spends as much ink on bather behavior as on residual chlorine.

Hygiene messages operators actually post and enforce

CDC's public messages are simple because the science is not. Teach staff to say them without apology:

• Do not swim with diarrhea. Especially not children in learn-to-swim, splash pads, and wading pools.
• Shower with soap before entering. A rinse shower plus a toilet stop cuts the nitrogen and fecal load that becomes combined chlorine and, worse, oocysts.
• Do not swallow the water. Crypto is an ingestion disease.
• Take bathroom breaks on a schedule. Swim diapers are not leak-proof; they are a dress code, not a barrier.
• Change diapers in the bathroom, not on the deck at water's edge.
• Check free chlorine and pH before the first patron, and keep checking.

A written diarrhea-exclusion policy is only as real as the front-desk person who turns a sick child away on a sold-out Saturday. MAHC-style codes expect that policy. Your AHJ may already require it.

The MAHC is a model, not your automatic law

The Model Aquatic Health Code is national guidance CDC publishes to help health departments write science-based rules for public pools, hot tubs, and splash pads. CDC develops it with the Council for the Model Aquatic Health Code (CMAHC). CMAHC members submit change requests, vote, and send results to CDC. For the 2024 edition, CMAHC handled 202 change requests and voted to pass 122 (60.4%).

Current public edition: 2024 MAHC, 5th edition

The most recent version is the 2024 MAHC, 5th edition, posted 12/16/2024. CDC archives earlier editions. The 2023 4th edition is the immediate prior public book; do not treat a 2018 or 2016 printout as current model language. When this guide cites MAHC section numbers (for example 6.5.3 contamination disinfection), it is teaching from the 2024 5th edition as public model text — still not your local ordinance unless the AHJ adopted that language.

Adopt none, some, or all

This is the operator trap that fails people on exams and on inspections:

• The MAHC is voluntary.
• A state, county, or city may adopt none of it, some of it, or all of it, and may edit what it adopts.
• OSHA, the Consumer Product Safety Commission (CPSC), and your building official still have their own statutes. MAHC does not erase them.
• Never tell a board, a parent, or an inspector that “CDC requires MAHC 6.5.3 in every U.S. pool.” CDC recommends. The AHJ requires.

In practice: Your county code still says 2 ppm free chlorine and a 30-minute formed-stool hold, and it has never mentioned Crypto CT 15,300. You still follow the county for what is legal. You still use CDC/MAHC numbers as the public-health reason to argue for a longer diarrheal closure, a UV unit on the splash pad, or a written contamination plan. Independent teaching keeps those two layers distinct.

Operator-relevant MAHC themes

You do not memorize the entire 2024 code. You do know the chapters that show up in Health and Safety domain work.

Contamination response plan

MAHC expects a written plan staff can execute: who closes the venue, how formed stool is distinguished from diarrhea, which clocks apply, where the log lives, and when the AHJ is called. Chapter 9 covered the first actions. The plan is the binder those actions live in. A plan that exists only in the director's head is not a plan on a holiday weekend.

Formed versus diarrheal stool

MAHC 6.5.3 splits disinfection the same way CDC fecal-incident recommendations do. Formed stool is treated as a Giardia-level event. Diarrheal stool is treated as a Cryptosporidium-level event. Vomit follows the formed-stool clock. Blood in a properly maintained venue is not treated as a public-health contamination equivalent to stool; operators may still close to satisfy patrons. Mixing those four events into one “shock and reopen in 20 minutes” script is how outbreaks and angry inspectors are born.

Hyperchlorination for Crypto

Hyperchlorination means raising DPD free chlorine (DPD-FC) on purpose and holding it for a concentration × time (CT) value, not dumping a bucket and hoping. For unstabilized diarrheal water, MAHC and CDC both use a Crypto CT inactivation value of 15,300 (mg·min/L) at pH 7.5 and about 77°F:

• 15,300 ÷ 20 ppm = 765 minutes = 12.75 hours
• 15,300 ÷ 10 ppm = 1,530 minutes = 25.5 hours

Time starts when the entire venue — not the gutter in front of the controller — has reached the target residual. Sample away from inlets. Conventional DPD kits do not read 20–40 ppm; use high-range strips or a dilution with chlorine-free water. After the CT is met, backwash to waste (do not return that water through the filter) and, where appropriate, replace media. CDC does not recommend testing the water for Crypto afterward: hyperchlorination destroys infectivity; it does not reliably destroy oocyst structure a lab might still see.

Secondary disinfection

UV and ozone can inactivate oocysts in the water that actually passes the chamber. MAHC treats secondary treatment as a design tool for increased-risk venues (water playgrounds, wading pools, other venues used by young children) and as an alternative to hyperchlorination after diarrhea if the system can theoretically drive oocysts below one oocyst/100 mL (MAHC 4.7.3.3.2.4). Secondary treatment does not replace a residual sanitizer in the basin. A bypassed UV lamp is interior decoration.

CYA and Crypto response difficulty

Stabilizer bonds free chlorine and slows Crypto inactivation. CDC and MAHC no longer pretend that the unstabilized 12.75-hour clock still works at outdoor CYA levels. If CYA is 1–15 ppm, CDC's current public table is:

• 20 ppm free chlorine for 28 hours, or
• 30 ppm for 18 hours, or
• 40 ppm for 8.5 hours.

If CYA is above 15 ppm, lower it to 1–15 ppm by partial drain-and-fill before those holds, or circulate through a qualifying secondary treatment, or drain the venue. MAHC Table 6.5.3.2.1 adds drain-percentage rows for higher starting CYA; those are model numbers, not a license to invent a third formula on a whiteboard. Do not invent a CT. Use the current CDC CYA protocol or an AHJ-accepted drain/secondary path.

Formed-stool plus CYA is a weaker data set. MAHC 6.5.3.1.1 doubles the formed-stool inactivation time when CYA is present. Follow the AHJ if it is stricter.

Operator training, chemical storage, bather load, hygiene facilities

MAHC's operations chapters expect a qualified operator, documented training, and chemical storage that keeps incompatibles apart in a ventilated space — the same story OSHA will own in section 11.2. Theoretical bather load in MAHC-style codes is not a marketing occupancy from the fire marshal's chair count. It is a water-surface and hygiene-fixture idea: toilets, showers, and diaper-changing stations have to exist in numbers that match the bodies in the water. A splash pad with one toilet and a birthday party of 40 toddlers is a Crypto amplifier even if the controller reads 3 ppm. Overcrowding is a water-quality failure, not only a customer-service problem.

Formed versus diarrhea: the operator table

Use this as the public CDC/MAHC concept table. The AHJ can be stricter. Chapter 9 still owns the first-minute close-and-net steps.

Worked example. Saturday 2 p.m., outdoor 90,000-gallon leisure pool, CYA 40 ppm, liquid chlorine only, a toddler has a diarrheal accident in the zero-depth. You already closed the leisure pool and the spray pad that shares the filter (Chapter 9). You netted what you could. You cannot run 20 ppm for 12.75 hours and call it done. CYA is above 15 ppm. Options that match current CDC/MAHC thinking: partial drain to pull CYA into the 1–15 ppm band, then 20 ppm for 28 hours (or the 30/40 ppm rows); or run a qualifying UV/ozone secondary treatment for the Crypto log-reduction the manufacturer and MAHC math support; or drain. You log the classification, the CYA, the path chosen, and the clock start. You do not reopen at 4 p.m. because the water “looks clear.”

Official public sources

• About the MAHC current edition (2024, 5th edition)
• 2024 MAHC, 5th edition (PDF)
• CDC fecal incident response recommendations (PDF)
• Outbreaks associated with treated recreational water, United States, 2015–2019 (MMWR)

When the AHJ, the MAHC, and a vendor's one-hour “Crypto shock” disagree, the AHJ is the law, CDC/MAHC are the public-health clocks, and the vendor is selling a product.