Explore the extensive list of frequently asked questions about perchlorate below, separated by issue area.
Basics
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Health
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How does perchlorate affect human health?
Studies show perchlorate’s direct effects on human health are limited to the thyroid gland. High levels of perchlorate can prevent the thyroid gland from absorbing iodide (which it uses to make hormones) from the bloodstream. This is called Iodine Uptake Inhibition, or “IUI.” This is not dangerous as the body automatically compensates for IUI. Further, perchlorate-induced changes to thyroid function have not been demonstrated in studies of humans exposed to perchlorate.
Is there an adverse health effect when perchlorate inhibits the thyroid gland’s ability to absorb iodide?
“Inhibition of iodide uptake (IUI),” as this effect is called, is a routine occurrence, caused by a number of factors in our diet and environment. The body naturally compensates if the thyroid can’t absorb its normal amount of iodide from the bloodstream. The thyroid itself has an enormous iodide reserve and can open additional “channels” to let more iodide in if needed. Most Americans eat a diet with more than twice the daily need for iodide.
Has perchlorate been shown to cause cancer in humans?
No. The National Academy of Sciences has confirmed it is unlikely perchlorate causes cancer. Numerous studies have shown NO evidence that perchlorate causes cancer in humans, even when consumed at levels far higher than any found in drinking water. The State of California has also stated that perchlorate does not pose a known cancer risk to the public.
What does it take to cause an adverse health effect?
In more than 60 years if study, no research has shown perchlorate causes adverse health effects. Theorectically, a sequence of three events would be required for perchlorate to have a potential adverse effect on health, each requiring higher doses.
First, perchlorate exposure must be high enough to prevent the thyroid from getting its usual amount of iodide. This may begin to happen at around 245 ppb of perchlorate in drinking water. Second, exposure must be high enough to overwhelm the body’s normal adaptive process, thereby lowering the amount of thyroid hormones in the body (scientific research indicates this does not occur at levels below 14,000 ppb). Third, exposre must be sustained long enough to reduce thyroid hormone levels for a long period of time. An adverse health effect would require daily consumption of more than 14,000 ppb in drinking water. To put this in perspective, more than 98% of perchlorate detections in U.S. water systems are below 10 ppb1– that’s 24 times lower than the recognized no effect level.
To put this all in perspective, at 10 ppb, a human would have to drink almost 740 gallons of water a day before a health risk could be possible.
1U.S. EPA unregulated contaminant monitring rule testing, 2001 (UCMR 1)
The NAS Committee says perchlorate is “unlikely” to cause cancer. What studies did the committee review in order to reach this conclusion?
The NAS Committee stated “it is unlikely that perchlorate poses a risk of cancer in humans.” (See page 145 of the NAS report, “Health Implications of Perchlorate Ingestion.”)
“There are no reports of the appearance of a new thyroid disorder, thyroid nodules or thyroid carcinoma in any patient treated with potassium perchlorate for hyperthyroidism. Iodide deficiency in the thyroid gland, a possible consequence of perchlorate administration or exposure, is not associated with an increase in thyroid cancer … In hyperthyroid patients treated with antithyroid drugs, there was no increase in thyroid cancer mortality” (See page 62 of the NAS report, “Health Implications of Perchlorate Ingestion.”).
“The committee concludes that the thyroid tumors in the (rat) offspring were most likely treatment related but that thyroid cancer in humans resulting from perchlorate exposure is unlikely because of the hormonally mediated mode of action and species differences in thyroid function.” (see page 12)
“In addition, EPA’s science policy document on the assessment of thyroid follicular-cell tumors notes that although there may be some qualitative similarities, there is evidence that “humans may not be as sensitive quantitatively to thyroid cancer development of thyroid-pituitary disruption as are rodents. The increased sensitivity may be due to marked species differences in the physiology of the thyroid gland. The EPA and IARC documents provide guidance for the evaluation of thyroid follicular-cell tumors based on mode of action (for example, tumors secondary to hormone imbalance).” (see page 145)
One of the animal studies of perchlorate looked at two generations of rats that were exposed to perchlorate in drinking water. A certain type of thyroid tumor known as a follicular cell tumor was identified in two of the rats given perchlorate. On page 145 of the report, the NAS Committee indicates that these types of tumors are not unexpected in rats when they are exposed to agents that affect the thyroid because “spontaneous thyroid follicular-cell adenomas can occasionally be observed in control rats of this strain and age.” In other words, thyroid tumors occur often in rats (especially Sprague Dawley rats as were studied here) even when they are not exposed to anything. The committee also notes that, in this regard, rats are much different than humans, stating “humans may not be as sensitive quantitatively to thyroid cancer development of thyroid-pituitary disruption as are rodents…The increased sensitivity may be due to marked species differences in the physiology of the thyroid gland.”
Rats are known to respond much more rapidly and to a greater extent to agents that affect their thyroid than do humans because of many physiological differences. When a rat is exposed to an agent that affects the thyroid, more thyroid stimulating hormone (TSH) is produced than in a human at a comparable dose, which causes a much more rapid production of thyroid cells. When cells multiply rapidly, the likelihood that cellular mutations will occur increases, which can lead to tumors. Since TSH levels are not affected in humans at equivalent doses, stimulation of the thyroid to produce more cells does not occur. The NAS states that these agents “can be assumed not to be carcinogenic in humans in concentrations that do not lead to alterations in thyroid hormone homeostasis.” (See page 145 of the NAS report, “Health Implications of Perchlorate Ingestion.”)
The EPA and the NAS Committee recommended a reference dose. What is a reference dose?
RfD is defined by the EPA as, “an estimate of a daily oral exposure to the human population (including sensitive subgroups such as children) that is not likely to cause harmful effects during a lifetime.”
See the definition of reference dose on the EPA website »
EPA’s definition is based on the assumption that exposure could occur throughout a lifetime and takes into account all stages of life. The definition also takes into account “sensitive subgroups,” such as pregnant women, infants, children and fetuses.
It is sometimes misunderstood to mean that doses above the RfD are unsafe. This is not correct. The RfD incorporates a number of safety factors to ensure the value is health-protective. When a dose exceeds an RfD value, it does not mean that an adverse effect will occur. Learn more about safe water from the EPA »
The RfD is not a regulatory standard —- it is one of the building blocks assembled into a regulatory standard, and should not be misinterpreted as serving the purposes of a regulatory standard.
How does EPA derive an RfD, and how did the NAS Committee derive one in the case of perchlorate?
Typically, EPA derives an RfD by starting with the highest dose at which no adverse effects can be observed (the No Observed Adverse Effect Level, or NOAEL). Learn more abou how the U.S. EPA sets RfD»
It is unprecedented to use the level at which no effect, even a mundane, biochemical effect, is observed (the No Observed Effect Level, or NOEL); however, this is the starting point that the NAS Committee used for perchlorate. The NAS Committee defined a NOEL as “the highest dose at which there are no statistically or biologically significant increases in the frequency or severity of any effect between the exposed populations and its appropriate control.” (See page 168 of the report, “Health Implications of Perchlorate Ingestion.”)
Where a NOAEL is used, uncertainty factors are customarily applied to the starting dose to account for scientific uncertainty and ensure the RfD protects public health. For example, to account for sensitive populations such as pregnant women, children, infants and fetuses, the EPA typically divides the NOAEL by an uncertainty factor of 3 or 10.
The NOEL dose that the NAS Committee selected as its starting point was 0.007 milligrams per kilogram per day and then divided this dose by a safety factor of 10 to account for sensitive populations. There is no precedent for this level of conservatism. The NAS Committee identified the NOAEL for perchlorate as 0.4 milligrams per kilogram body weight per day, stating that, in adults, “sustained exposure” (i.e., several months or longer) to more than 0.4 milligrams per kilogram of perchlorate per day is likely required before any harm could occur.
Alternatively, if an uncertainty factor of 3 or 10 for sensitive populations was applied to the NOAEL the reference dose could have set at 1,400 to 4,655 ppb in drinking water.
What is a NOEL?
The NAS Committee defines a NOEL as “the highest dose at which there are no statistically or biologically significant increases in the frequency or severity of any effect between the exposed populations and its appropriate control.” on page 168 of the report, “Health Implications of Perchlorate Ingestion.” The NAS committee states that a dose of 0.007 milligrams per kilogram of body weight per day is the NOEL. Using standard default EPA conversion practices, this is equivalent to 245 ppb in drinking water.
What are “uncertainty” or “safety” factors?
As with most issues in life, health and safety, decisions must be made absent absolute certainty. “Uncertainty” or “safety” factors account for a lack of scientific data or to address possible variability in population sensitivity when deriving a safe dose level.
Is it appropriate to apply a safety factor to a NOEL? Is there a precedent for this? Are there scientific arguments against it?
Applying a safety factor to a NOEL is unusual and no precedent has been identified.
The NAS Committee acknowledges its recommendation “differs from the traditional approach.” (See page 15) Moreover, the NAS Committee does not provide supporting precedent to justify applying a safety factor to this kind of dose. In this instance, the NAS Committee recommends a reference dose more than 570 times lower than the dose it concludes does not cause adverse effects in humans.