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How Safe is Seattle's Water?

Cryptosporidium in Seattle's Water Supply

Summer, 1999

Seattle Public Utilities reports that the cysts that can cause cryptosporidiosis (sometimes referred to as "crypto") have been found periodically in the city water supply. Symptoms include diarrhea, abdominal cramps, bloating, headaches, nausea, vomiting and low-grade fever. While there have been no known waterborne outbreaks of crypto in the area, infection can be severe enough to cause death in persons with CD4 counts below than 200.

The cysts are extremely difficult to remove from the water supply. They can pass through filters with a pore size of only 1 micron (1/25,000 inch) and are not killed by chlorination, Seattle's primary method of water treatment.

One way Seattle helps prevent contamination of the water supply is by restricting public access to its watersheds. These watersheds are home to natural wildlife, however, which can be sources of the disease. Some new filtration and treatment systems will be in place as early as 2000, and work should be completed by 2004.

How to Reduce Your Exposure

The simplest and most effective way to ensure safe drinking water is to boil it: 1 minute at sea level and 3 minutes at altitudes above 6,500 feet. Filtering can reduce risk of infection but will not eliminate it. The Centers for Disease Control recommends only three types of filtration:

  • Microstraining with filters that remove particles between 1 micron and 1/10th of a micron in size. These measurements must be labeled "absolute" 1 micron, not "nominal" 1 micron.

  • Reverse osmosis, a process that uses pressure to move water through a semipermeable membrane.

  • Filtration using equipment that specifically states it meets National Sanitation Foundation (NSF) Standard 53 for "cyst reduction."

Do not assume that all bottled water is safe. Bottled water that has been distilled or filtered by reverse osmosis is usually OK.

Exposure to cryptosporidium can occur from contact with soil, recreational pools, lakes, and barnyard animals. Person-to-person infection can occur from sexual contact or contact with any fecally contaminated surfaces. Beyond safe drinking water, good hygiene is the best defense against the disease. This includes washing hands before eating, avoiding raw or undercooked foods (including unpasteurized juices), and wearing gloves when gardening.

According to Seattle Public Utilities, the general population is not at risk and not everyone who is exposed to the cysts becomes ill or displays symptoms. For the immuno-compromised who do get the disease, however, there is no known effective cure.

If you think you are at risk or have more questions, call the Seattle-King County Department of Public Health HIV/STD Hotline at (206) 205-7837. More information is also available through the Seattle Public Utilities' water quality website at and the Centers for Disease Control's website at Or call the STEP TalkLine at 206-329-4857.

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This article was provided by Seattle Treatment Education Project. It is a part of the publication STEP Perspective.