Short definition
Cryptosporidium is a single-celled parasite (protozoan) whose oocyst — its dormant, transmissible form — has a thick wall that resists standard chlorine disinfection. To inactivate it, water utilities use UV light, ozone, or absolute sub-micron filtration. Boiling for at least 1 minute (3 minutes above 6,500 feet elevation) reliably kills it.
What it is
Cryptosporidium is a microscopic protozoan, 4 to 6 microns in diameter, that infects mammals — including humans — through the fecal-oral route. The organism’s transmissible stage (the oocyst) is encased in a thick cell wall that survives in water for months and shrugs off the chlorine concentrations used in normal drinking-water treatment.
The numbers tell the story. To get 99% inactivation of E. coli with free chlorine at 1 ppm, contact time is on the order of seconds. To get the same inactivation of cryptosporidium, you’d need around 9,600 mg·min/L — roughly 7 days of contact at typical residual concentrations. Standard chlorination, in practice, doesn’t kill it.
What does work: physical removal by filtration to sub-micron pore size (NSF/ANSI 53 absolute 1-micron rating blocks the oocyst), UV light (a 12 mJ/cm² dose gets 4-log inactivation), or ozone. And boiling — at least 1 minute at sea level, 3 minutes at higher elevations.
Why it matters to a homeowner
Two reasons cryptosporidium matters in Washington specifically.
First, regional water supplies. Seattle’s Cedar/Tolt watershed is unfiltered and uses UV plus chlorine plus ozone — the treatment train is built around the fact that crypto is in surface water and chlorine alone won’t handle it. Tacoma’s Green River supply is filtered. Bellingham, Everett, and other utilities each manage their own combination. The reason they spend the money: the alternative is a Milwaukee-1993-scale outbreak (400,000 illnesses, 100+ deaths from a single contamination event), which is the historical reference that drives utility crypto-treatment design.
Second, private wells. A shallow well or a cracked well casing can let surface water (which often carries crypto) into a private supply. Standard well-water tests for coliform and nitrates don’t detect crypto. Surface-influenced wells need either NSF 53 absolute 1-micron point-of-use filtration or whole-house UV — chlorine alone is not enough.
A common homeowner mistake worth correcting: shock chlorination of a well kills coliform bacteria, not crypto. After shock-chlorinating a surface-influenced well, the cyst risk remains.
When you’ll encounter this term
- A boil-water advisory from your utility (rare in WA but possible after main breaks or contamination events)
- Shopping for a point-of-use water filter — read the certified-claims list for NSF 53 cyst rating
- A rural well user with a shallow well or seasonal yield variation
- Backcountry water sources in the Cascades or Olympics — crypto + giardia are present in essentially all surface water
- A family member with reduced immunity (chemo, transplant, HIV+) — extra precaution warranted
Common variants and disambiguation
- Cryptosporidium vs. giardia. Both are chlorine-resistant protozoa. Giardia is somewhat more chlorine-susceptible and a different size (8–12 microns). The same NSF 53 absolute 1-micron filter handles both.
- Cryptosporidium vs. coliform. Different organism class; different test; different treatment. Crypto is not detected by coliform tests, and a clean coliform test does not rule it out.
- Crypto in tap water vs. crypto in pool water. Pool advisories during outbreaks recommend hyperchlorination; not the same as treating drinking water.
Mitigation hardware
- NSF/ANSI 53 absolute 1-micron filter (point-of-use under-sink): $30–$80 cartridges; $200–$600 installed system. Blocks crypto cysts.
- Whole-house UV at point-of-entry: $400–$1,200 hardware + $200–$500 install. Kills crypto when properly sized and maintained (annual UV bulb replacement is essential).
- Boiling during a boil-water advisory: free; ≥1 minute at sea level, 3 minutes above 6,500 feet.
- Backcountry filters (Sawyer, Katadyn, MSR) with 0.1–1 micron absolute rating: $25–$120 — for camping/wilderness.
Common failure modes (for treatment systems)
- Filter that says “removes bacteria” but isn’t NSF 53 cyst-rated. Generic carbon filters do not block crypto reliably. Check the certified-claims list.
- UV system with a dead bulb. UV only works when the lamp is producing 254-nm output. Replace bulbs annually per manufacturer.
- Surface-influenced private well treated like a normal groundwater well. Coliform tests miss crypto; treat surface-influenced supply with UV + sub-micron filter regardless of bacterial result.
- Conflating “chlorinated” with “safe.” Public utilities supplement chlorine with UV or filtration specifically because chlorine alone doesn’t handle crypto.
Washington note
Washington’s Group A surface-water utilities comply with EPA’s Long Term 2 Enhanced Surface Water Treatment Rule (LT2), which mandates monitoring for crypto and additional treatment when source water exceeds thresholds. Seattle Public Utilities operates Cedar/Tolt unfiltered with UV + chlorine + ozone. Tacoma Water filters Green River source. Bellingham, Everett, and others each have their own treatment train.
WA Department of Health issues boil-water advisories when crypto contamination is suspected. Boiling — not just chlorination — is the homeowner-side response. Boil ≥1 minute at sea level (3 minutes above 6,500 feet) for drinking, brushing teeth, ice, and dishwashing rinses.
For private well users with surface-influenced supplies (shallow wells, cracked casings, recent flooding events), the prudent default is point-of-use UV plus NSF 53 absolute 1-micron filtration, regardless of how clean the coliform test looks. Crypto + giardia + viral infiltration risks aren’t visible in standard tests.
FAQ
Does my home water filter remove cryptosporidium?
Only if it’s certified to NSF/ANSI 53 with a “cyst” reduction claim, or NSF/ANSI 58 (RO membrane). Generic carbon-block filters typically do not. Read the certified-claims list on the filter packaging — marketing copy isn’t enough. If it doesn’t say “cyst” or “cryptosporidium” specifically, assume it doesn’t.
Will boiling water kill cryptosporidium?
Yes. Boiling for at least 1 minute at sea level reliably kills cryptosporidium, giardia, coliform, and most other waterborne pathogens. Above 6,500 feet of elevation (Cascades passes, eastern WA elevations), boil for 3 minutes to compensate for the lower boiling temperature.
Why doesn’t chlorine kill cryptosporidium?
The oocyst — crypto’s dormant form — has a thick, multi-layered cell wall that resists chemical disinfection. Standard chlorine residual concentrations and contact times in drinking-water systems just don’t penetrate it. Achievable utility-scale chlorination would need contact times of days, which isn’t practical, so utilities use UV or ozone instead.