(CNN)The Environmental Working Group reported Tuesday that dangerous levels of chromium-6 contaminate tap water consumed by millions of Americans. This is the carcinogenic chemical featured in the true story turned Hollywood movie "Erin Brockovich,"
For its report, the independent environmental advocacy group examined evidence from water systems throughout the nation.
What level of chromium-6 in drinking water is considered dangerous?
The US Environmental Protection Agency has set the federal standard for total chromium in drinking water at 100 parts per billion (ppb). The EPA has never set a specific limit for chromium-6, one form of chromium, a naturally occurring element found in rocks, animals, plants, soil and volcanic dust and gases. Chromium-6 is rare in nature though it is produced by industrial processes. Airborne and large quantities of chromium-6 are known to be toxic.
In fact, the US Occupational Safety and Health Administration states that high levels of airborne chromium-6 can cause cancer; the agency requires companies to protect their employees from workplace exposure.
However, there is uncertainty about chromium-6 in drinking water. Scientists disagree about what exactly a safe amount for water is and what the possible long-term consequences may be when people ingest it through water.
The Environmental Working Group used two separate standards when deciding whether existing chromium-6 levels in community water systems might be dangerous.
The first standard is the public health goal of 0.02 parts per billion set by California"s Office of Environmental Health Hazard Assessment. This level, which would pose only "negligible risk over a lifetime of consumption," is endorsed as "safe" by the Environmental Working Group because it addresses concerns about fetal, infant and child exposure to the chemical. What is nontoxic for an adult may not be so for a developing baby.
The group also looked at the actual legal limit adopted by California regulators, even though the group"s scientists consider this too lenient a standard: 10 parts per billion.
By the stricter standard, the Environmental Working Group found the tap water of 218 million Americans has dangerous levels of chromium-6.
By the more lenient standard, the group found the tap water of 7 million Americans has dangerous levels of chromium-6.
Where was chromium-6 found?
The report found chromium-6 in almost 90% of the water systems sampled across the nation. Oklahoma, Arizona and California had the highest average statewide levels, though not necessarily above the lenient standard.
Among major cities and metropolitan areas, Phoenix had the highest average level, at almost 400 times the California health goal: 7.853 ppb. St. Louis County in Missouri (1.258 ppb) and Houston (0.747 ppb), relative to other urban areas, had high levels as well.
Following these were Philadelphia (0.388 ppb); Dallas (0.274 ppb); California"s South Coast Water District,including Capistrano (0.223 ppb); Columbus, Ohio (0.207 ppb); and the Las Vegas Valley Water District (0.203 ppb).
Chicago, San Antonio, the Washington Suburban Sanitary Commission in Maryland, Fairfax County in Virginia and Cleveland all ranged between 0.10 ppb and 0.20 ppb.Finally, Miami-Dade in Florida; San Diego, Laguna Beach, and Alameda and Contra Costa counties in California; New York City; Baltimore; Denver; and San Juan, Puerto Rico, all ranged between 0.035 ppb and 0.085 ppb.
The Environmental Working Group report provides more specific information.
Who is testing for chromium-6?
Water systems are required to test for total chromium, which includes both chromium-6 and chromium-3, a naturally occurring form that is an essential nutrient for the body. Total chromium found in water samples must be below the current federal standard of 100 ppb.
The EPA does not regularly test water systems of communities with fewer than 10,000 people, relying on random testing instead.
Still, no matter how large or small a community water system, anytime total chromium in tap water exceeds the federal standard, local residents must be notified.
Yet chromium-6 and chromium-3 can convert back and forth in water and in the human body under the right chemical environment. The EPA regulation of 100 ppb assumes any measurement of total chromium can turn into 100% chromium-6.
This regulation is a concern to some scientists. Based on the best available science at the time, the EPA regulation was established more than two decades ago, in 1991. With rapid advancements in science, some experts believe the rule may be outdated today.
The EPA is reviewing scientific data, including the results of a long-term animal study that suggests chromium-6 may be a human carcinogen if ingested. It expects to complete its review sometime next year, and at that time, the agency will allow public comment. Following this, the agency will consider a decision to revise its chromium limit.
If you are worried for yourself and your family, you might protect yourself by the steps below.
How do I test my water for chromium-6?
Most commercial do-it-yourself home test kits do not offer a chromium-6 test.
Whether you have a well or municipal water, your best option for testing would be either a water treatment professional or an EPA-certified lab, which has been evaluated as technically competent to provide accurate data. The Water Quality Association, an industry organization for water treatment and delivery companies, can help find professionals and labs in your area.
For additional information about nearby water testing, you can also use the state and province list from the Water Systems Council, another industry group.
You might also find it helpful to visit your local health department"s environmental health division to ask for information about what has typically been found in local groundwater, suggests NSF International, a product testing, inspection and certification organization. Your health department might also provide you with more information about testing services as well.
Can I test myself and family members to see if we"ve been exposed to chromium-6?
According to study results presented at a scientific guidance panel in California, chromium can be measured in a wide variety of biological samples, including blood, urine, saliva, hair, breast milk and joint fluid. Absorbed chromium is secreted in urine. Blood and urine measurements are considered most reliable for detecting elevated levels in the body.
Within red blood cells and other parts of the body, chromium-6 may be reduced to chromium-3, a nontoxic form of the chemical. Yet differences between individuals exist: Some people may more readily and more successfully convert the chemical to the harmless form. Elevated levels in either blood or urine may indicate exposure, but other factors complicate the interpretation of results.
"To interpret elevated chromium urinary and/or blood levels, additional information, such as from an exposure questionnaire, is necessary," wrote the authors of the report.
In other words, it"s complicated. Still, if concerned, you can discuss getting a chromium test -- a simple blood test that is sent to a lab -- with your doctor.
Can I filter chromium-6 from my water?
According to the Environmental Working Group, chromium-6 can be filtered from water by certified products which include one simple pitcher and 12 under-the-sink reverse osmosis units. Suggested water filters can be found here.
Reverse osmosis uses a semipermeable membrane as a filter to remove ions, molecules and large particles from water. Generally, these purification units can be found in hardware stores and installed under the sink. Water is forced across the membrane, effectively blocking chromium-6 and other harmful substances such as lead and arsenic.
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