As our investigation found, when scientists and doctors do look, the connections they’ve found underscore the need to protect public health by reducing Americans’ exposure to this potent toxin.But it's not that simple as the EPA points out:
The behavior of arsenic in the body is very complex. After absorption, inorganic arsenic can undergo a complicated series of enzymatic and non-enzymatic oxidation, reduction, and conjugation reactions. Although all these reactions may occur throughout the body, the rate at which they occur varies greatly from organ to organ. In addition, there are important differences in arsenic metabolism across animal species, and these variations make it difficult to identify suitable animal models for predicting human metabolic patterns. (1)So with that in mind, the EPA has moved forward to significantly change the estimated carcinogenic potency of arsenic, which the Texas Commission on Environmental Quality (TCEQ) believes "already has a relatively high SFo." (2)
What's the EPA using to support this change? The TCEQ points out:
USEPA used lung and bladder mortality data from Morales et al. (2000) for the dose-response assessment for the final draft SFo.And that "Morales et al. uses these mortality data to calculate standardized mortality ratios (SMRs) and notes:"
“Although the computed SMRs display a large amount of noise, there appear to be higher SMRs at high exposure levels compared to exposures in the lower range, especially for bladder and lung cancer.”Wait just a dang minute, TCEQ exclaims:
To say that there is “noise” in the SMRs over the eight exposure categories is an understatement.What is the TCEQ basing this on? Well back to square one, it's all about the dose-response curve and the SFo derived from it. If we are going to except the 17 fold increase in potency then we must accept the dose response curve used to determine the slope. The TCEQ notes this, stating:
Dose-response is the cornerstone of toxicology, but the lung and bladder mortality data (SMRs) from Morales et al. (2000) provide a poor basis for dose-response assessment as a dose-response is not apparent and not monotonic.If you recall, the slope is derived from the line representing the dose and response and is based on the assumption that every dose poses a risk. It is all predicated on a dose-response where the higher the dose the greater the risk. With that in place you can get a line and a slope:
That line though, is dependent on a linear progression - higher the dose, higher the risk or occurrence. But that's not apparent in the Morales et al. data the EPA is using according to the TCEQ:
For bladder cancer, the dose-response data from Morales et al. (2000) and used by USEPA do a poor job of characterizing the shape of the dose-response curve, as can be seen from the figure below (line added for emphasis).What does the dose-response curve look like for bladder cancer? Take a gander at this:
The ability to fit a line through data points does not necessarily mean that the underlying data adequately define the shape of the dose-response curve, including the critical low dose region. Based on the above considerations, the underlying data modeled by USEPA provide a poor basis for dose-response assessment.The 17 fold increase is based on the Morales et al. data. The SFo the EPA proposes is based off of a line that used a dose response curve that looks the one above. How much confidence in that SFo should we have?
So why does Keeve Nachman, the Johns Hopkins scientist, tell Consumer Reports:
The [EPA] proposal "suggests that arsenic's carcinogenic properties have been underestimated for a long time and that the federal drinking-water standard is underprotective based on current science."You'll have to ask him that. I suspect he, like a lot of others, just accepts what others professionals have to say as long as it follows their established way of thinking. Arsenic is toxic ergo any amount in the water is unhealthy.
But "current science" has not brought forth anything that can support lowering the current 10 ppb in drinking water. So what's the problem? Well the damage has already been done. When Consumer Reports writes:
For known human carcinogens such as inorganic arsenic, the EPA assumes there's actually no "safe" level of exposure, so it normally sets exposure limits that include a margin of safety to ideally allow for only one additional case of cancer in a million people, or at worst, no more than one in 10,000. For water with 10 ppb of arsenic, the excess cancer risk is one in 500....it has drawn a line in the sand for the public. The risk of cancer now becomes one in 500 for water the EPA claims has been telling us is "safe."
But that risk of one in 500 is based on a slope generated from a squiggly line akin to a path from the Family Circus.
That Morales et al. dose-response curve is what helped generate a SFo that is 17 times more potent than what we currently accept.
Next post: Apples, Arsenic, and Risk - Part 5: Calculating one in 500