Risk Assessment Can Never Die
Sometimes, rationality is unavoidable. Arsenic is classified by the International
Agency for Research on cancer as “Group 1: The agent (mixture) is carcinogenic
to humans”. Not only that, the primary
route of exposure associated with carcinogenesis is from drinking water AND
arsenic is a common contaminant in drinking water, even in the United States. Since arsenic is not intentionally added,
getting rid of it costs money. How much
money? Well, that depends on how much of
the arsenic you want to get rid of. So,
at the turn of the century, the EPA needed a real risk assessment that could
justify the expenditure of real money.
But there was a hitch; agency guidelines then and now more
or less forbade the creation of scientifically plausible risk estimates. Fortunately, that was also a problem money
could solve: The agency hired outside help to characterize the dose-response
relationship (Morales et al, 2000). While the agency guidelines were acknowledged,
they weren’t followed. There was no
default option. Several different model
options were explored. Most of them
weren’t very plausible, but at least one of them was – which given the
circumstances is quite admirable.
Furthermore, that is the model the economic analysis primarily relied
upon, which is very nice too.
The Cost-Benefit Analysis
In addition to making use of cost estimates for reducing
arsenic levels in municipal water supplies of varying sizes, the cost-benefit analysis
developed to support the 2001 arsenic drinking water rule (EPA, 2000) also
estimated the reduction in risk that could be expected by keeping arsenic concentrations below a
series of possible regulatory levels.
Like the dose-response analysis, the economic analysis was done under
contract outside the agency. However,
the exposure assessment portion of the risk assessment relied upon he same
values used by the agency for other evaluations. Summary results of risk reduction estimates (from
section 5.4.1) are as follows:
Arsenic Level
(µg/L)
|
Bladder Cancer Cases Avoided Per Year
|
Lung Cancer Cases Avoided Per Year
|
3
|
28.6 – 76.8
|
28.6 - 61.5
|
5
|
25.6 – 55.7
|
25.6 – 44.5
|
10
|
18.7 – 31.0
|
18.7 – 24.8
|
20
|
9.9 – 10.6
|
8.5 – 9.9
|
In a separate analysis, the cost of implementing water
purification systems that would achieve each specified level was estimated. When calculated on a per household basis, the variation
between purification costs was far more dependent on the size of the system
than the target level (see section 6.3.3).
In small systems, per person costs (and water bills) were estimated to
increase by over $300 per year. In large
systems serving over a million people, even the most stringent standard was
estimated to carry a cost of less than $10 per person. In addition, the relative cost of achieving lower
arsenic levels only increased significantly with systems with more than 1
million people. These differences are a
result of two main factors. First, there
is an economy of scale that makes purification cheaper when to costs is borne
by more users. Second, systems with groundwater
sources are more likely to need treatment in order to attain a given level; and
smaller systems are more likely to come from groundwater rather than surface
water.
Cost-benefit ratios were calculated on a national
basis. This required assigning a monetary
value to each cancer case avoided (aka the “Value of a Statistical Life”; see
section 5.4.2) and then comparing the monetized anticipated benefits with the
anticipated costs. Since a benefit to
cost ratio of 1 or greater was achieved at either 10 or 20 µg/L, but not 3 or 5 µg/L, a standard of 10 µg/L
was adopted as the Maximum Contaminant Level.
An Unsafe Level
Risk assessments are approximately correct, at best. Cost-benefit analyses are acceptable, at
best. The analysis conducted for the
2001 Arsenic Drinking Water Rule was far from perfect, but it was good enough
to justify the regulation, and that wasn’t easy. It only happened because the analysis did NOT
follow current (then or now) agency guidelines that discourage the generation
of credible or even plausible risk estimates.
One of the consequences of that is that the MCL for arsenic in drinking
water is not “safe”, at least in the sense that the term is operationally
defined by the Safety Assessment paradigm.
Nor does the MCL achieve the risk target of 10-4 (1 in 10000)
that the EPA typically uses the gauge risks for contaminants in drinking water
(EPA, 2012). Therefore, using the
drinking water standard to judge arsenic levels in juice or wine
is wholly unjustified. Furthermore, it
is not at all clear that an FDA standard for bottled water should be the same
as the EPA standard for tap water; perhaps it would be more economically
feasible to just purify water that is actually drunk, as opposed to also
purifiying the water is also used for bathing, washing dishes and clothes, and
watering the lawn.
Fifteen years later, just about every component of the economic
analysis conducted in 2000 is in need of some revision. But still, the risk assessment can serve as a
useful template for an updated analysis.
In fact, that’s one of the great attributes of risk assessments: They
are far easier to improve than safety assessments. However, one aspect of the
economic analysis is perhaps in need of restructuring; estimating risk-benefit
ratios on a national basis seems ill-advised.
In particular, consider the most extreme differences in costs per household:
Arsenic Level
(µg/L)
|
Cost Per Household
Systems with <100 Persons
|
Cost Per Household
Systems with > 1,000,000 Persons
|
3
|
$317
|
$7.41
|
5
|
$318.26
|
$2.79
|
10
|
$326.82
|
$0.86
|
20
|
$351.15
|
$0.15
|
Even though these numbers aren’t exactly comparable for a
number of reasons, it seems obvious that a a cost-benefit analysis tailored to
individual water systems would identify different optimum MCLs for different
systems. More specifically, it seems
likely that a lower MCL could be justified for large systems, while perhaps even
20 µg/L isn’t worth it for small systems. If the Federal government were paying for it,
then maybe you could call it Environmental Justice. But they aren’t, and therefore a national
standard may be a bad deal for everybody.
[Similar arguments can and have been made for minimum wage standards and
health care].
References
EPA (2000). Arsenic
in Drinking Water Rule Economic Analysis. EPA 815-R-00-026.
EPA (2012). 2012
Edition of the Drinking Water Standards and Health Advisories. EPA
822-S-12-001
Morales KH, Ryan L, Kuo TL, Wu MM, and Chen CJ (2000). Risk of internal
cancers from arsenic in drinking water. Environ
Health Perspect. 108: 655–661.
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