of 153 STRENGTH

This is defined by the size of the association as measured by appropriate statistical tests. The stronger the association, the more likely it is that the relation of "A" to "B" is causal. This criterion has also been adapted for toxicological risk assessments to consider both the incidence and severity of effects.

The TVAT has examined the criteria of “strength” from two perspectives: the strength of the correlation between an adverse health effect and presence on a Hanford tank farm and the strength of the potential exposure.

Relative to the strength of the correlation between the tank vapor exposure potential and adverse effects, there is essentially a perfect correlation between the workers having an adverse acute inhalation health effect and being physically present on a tank farm. That is, from what the TVAT can determine, there is a one-to-one concordance (R=1.0) between being on the tank farm and the reported occurrence of acute symptoms. This is not to say that every person on the farm would have an adverse effect; indeed, the bolus hypothesis predicts that relatively few persons on any farm on any particular day would have an adverse effect. This criteria measure only points to the fact that the affected persons were invariably within or proximate (within 25’ of the fence) to the farm.

With regard to the strength of the exposure potential, examination of the tank vapor head-space and vent concentrations show extremely high concentrations of many of the chemical species present relative to the potential of these vapors, if inhaled, to cause significant acute irritation of the upper respiratory tract. Indeed, this irritation potential of the vapors in the tanks head space (and vents) is generally accepted by all, including the IH professionals at Hanford to be very potent relative to their ability in undiluted concentrations to cause upper respiratory irritation. That is, the inhalation of a few breaths of Hanford tank vapor at essentially undiluted vent concentrations is considered, by essentially everyone, to be strong enough to account for the noted symptoms reported by workers. This conclusion is supported by unsolicited descriptions provided by numerous interviewed workers of immediate and severe onset of irritation or other acute symptoms. The TVAT also believes that the incidence of such effects among workers (based on discussions with workers and leadership in focus groups and individual interviews) goes well beyond what would be reasonably considered as not related to actual exposure events. Thus, this is a strong association.

DOSE-RESPONSE RELATIONSHIP

An increasing amount of exposure increases the risk. If a dose-response relationship is present, it is strong evidence for a causal relationship.

There is little doubt that the chemical species present in the Hanford tank head-space and vents represent a risk of at least acute severe upper respiratory irritation upon inhalation by workers. Acute exposure to these chemicals would clearly and strictly adhere to a dose-response relationship in which monotonically increasing dose or exposure would be associated with increasing response. This increase will occur with both increasing time at a constant concentration and increasing concentration during any constant time interval. The classic description of this dynamic two-dimensional relationship for acute inhalation exposure events is given in the following equation:

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)

ln(C

t

b

a

Y





Yp = Probit value corresponding to p% response. (e.g., Y50 is the probit value that corresponds to 50% response, and is equal to 5)

ln = natural log base e

C = breathing zone concentration (ppmV or mg/m3) a, b, n = coefficients

t = time (minutes)

In this case the response would be the probit value that predicts the proportion of the exposed population with the adverse health effect, or the percentage with at least severe upper respiratory irritation that will increase as a function of both concentration and time. That is, as either

exposure time or exposure concentration is increased, the effect and the adversely affected proportion of the exposed population will increase. The above equation only predicts the proportion of the exposed population responding; however, a different but conceptually similar model, given the right data input, would predict an increasing level of severity of effects in individuals at any concentration and time.

In this case, our hypothesis is that the time frame of exposure is very short (one second to a few seconds); thus the concentration is concomitantly high enough to achieve a response in

essentially everyone (Yp > 9) who is exposed. Inhalation of a significant proportion of the tank

vent concentration is fully expected to cause a severe acute toxic effect in everyone. CONSISTENCY

The association is consistent when results are replicated in studies in different settings using different methods. That is, if a relationship is causal, we would expect to find it consistently in different studies and among different populations.

It is our full expectation that even brief inhalation exposure to tank vapors at or near the level of head-space or vent concentrations will consistently cause workers to experience significant effects. In our interviews of tank farm personnel we learned of a term used on the site to describe such exposure events, “go down in the farm”. This was a phrase provided by a manager during one of our focus session in describing the almost instantaneous response to a breath or two of noxious air on the tank farm often resulting in a severe response of a worker “going down” to one knee or momentarily losing their balance.

The reports of effects of these exposures from numerous individuals in discussions held with various focus groups and individuals has been remarkably consistent relative to their quick onset and resulting upper respiratory symptoms and these facts are consistent with the hypothesis (see plausibility below).

PLAUSIBILITY

The association agrees with currently accepted understanding of pathological processes. In other words, there needs to be some theoretical basis for positing an association between a vector and disease.

Given the workers’ reports of exposure events and working backward from the adverse health effects toward potential sources and time frames of exposures causing the effects presents a plausible hypothesis. The TVAT postulates that vapors are coming out of the tank in high (bolus)

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