The_400_Level_What_Is

The 400 Level – What Is It?

How should it be interpreted?

Alfred A. Brooks

OREJC Approval 9/6/05

7/4/05

http://home.comcast.net/~brooks50/The_400_level.htm

DRAFT

The 400 level is of course the remediation level, 400 ppm for mercury, finally adopted for the excavation and removal of the Lower East Fork Poplar Creak (LEFPC) floodplain soils. But what does it mean for health purposes. Does it mean that all levels above it are unhealthy? Does it mean that all lower levels are safe for anyone? To answer these and other questions required an understanding of the remediation goal setting process as defined by the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA).

There are several possible participants in the remediation goal setting process:

Participant	Role	Scope
Department of Energy (DOE)	Responsible party	For remediation
Environmental Protection Agency (EPA) [1]	Regulatory agency	For the environment
Agency for Toxic Substances and Disease Registry (ATSDR)[2]	Non-regulatory agency	For public health
Tennessee Department of Environment and Conservation (TDEC)	Overview agency	All operations
Stakeholders	The public	All remediation

Notes: The Agency procedures may be found at the following links:

[1] EPA: Risk Assessment Guidance for Superfund Volume 1 - Human Health Evaluation Manual ( Part B, Development of Risk-based Preliminary Remediation Goals); EPA Pub 9285.7-01b, Dec 1991 - http://www.epa.gov/oswer/riskassessment/ragsa/index.htm

[2] ATSDR: Minimal Risk Levels (MRLs) for Hazardous Substances http://www.atsdr.cdc.gov/mrls.html

Exposure/Dose/Response Model

There are two principal portions in the Exposure/Dose/Response model:

One or more exposure routes (soil, air, water, etc.) which transport contaminants from the environment into the human system (blood). Adsorption by the human gut, lungs and skin is included in the exposure route analysis.
Note: Radiation exposes the body tissues directly and does not involve the blood except as an exposed tissue.
The response of the human body to the presence of the contaminant in the blood.

Both of these steps involve processes for which data is lacking and must be assumed or involves extrapolation from test animals to humans. If these approximations are too conservative every thing will be remediated but lack of any safety factor may expose humans to harm. Some of the assumed safety factors used by EPA are believed to be conservatively large by many. The subject is under great debate among professionals and new data is always available but even more is still missing.

Differing Methodologies for Comparisons Values in Dose and Risk Assessment

The ATSDR and the EPA differ in their dose and risk assessment. These differences make it difficult for the public to understand the differences in their results. The most significant differences in the comparison values are described below:

Difference	ATSDR	EPA
Evaluation Objective	To evaluate threats to public health at a discernable level.	Establish unquestionably safe cleanup levels for environmental and public health which justifies the use of biased input data.
Comparison Value [1]	Minimum Risk Level (MRL)[2]	Reference Dose (RfD)[3]
What it is	A mildly conservative estimate of a human dose which will not cause observable public health effects. Comparison levels may be derived for screening and then refined for a final opinion.	A (sometimes excessively) conservative estimate of a human dose which unquestionably will not cause observable health effects in the most sensitive individuals. A basis for a conservative cleanup level.
What it is NOT	An estimate of a sharp demarcation between safe and unsafe environmental levels. A clean-up level	An estimate of a sharp demarcation between safe and unsafe environmental levels.
Data Sources for Comparison Levels and Exposure Routes	Exposure and Dose Data plus Dose/Response Data for specified symptoms.	Exposure and Dose Data plus Dose/Response Data for specified symptoms.
Safety factors applied	Mild Discussed openly	Large; sometimes excessive Buried in data assumptions Appear as excess risk
Modus Operandi	Weight of Evidence	Computational, definitive
Conservatism	Mild	Large; sometimes excessive
Comments	a) By making weight of evidence decisions about both animal and human (if any) data, ATSDR avoids the need for scarce explicit dose/response/ risk data b) Safety factors are readily apparent.	a) Method requires data for dose/response and risk much of which is missing and must be assumed. b) By making conservative assumptions on each missing input datum, the final degree of conservatism is buried in the risk computation.

Notes:

[1] Other authoritative levels or doses, e.g., drinking water standard, may be adopted as the comparison value. The terminology is different for exposure to radiation or radioactive contaminants but the principles are similar.

[2] ATSDR uses screening levels and based on weight of evidence can declare a dose that that is somewhat higher than the comparison value to be safe. Or they may, by further study, determine the comparison value more accurately.

[3] The EPA RfD is considered to be a maximum permissible value which should not be exceeded in spite of a large conservative bias.

It is important to note that EPA and ATSDR do not estimate the same parameter nor for the same purpose and thus should not be expected to produce the same result. The differences may be large. These results should be considered as a regulatory level (EPA) or advisory level (ATSDR) and NOT a level above which a serious public health threat is expected. The safety factors range from about 10 to 1000 or more.

The CERCLA Approval Process

Loosely and simplistically, the CERCLA approval process goes like this:

After appropriate study and investigation, DOE, interacting with EPA, TDEC and stakeholders, prepares a remediation plan. A calculation of the effects of human exposure and environmental exposures by DOE and EPA is included using the EPA default parameters when data is not available. Then, if not earlier, ATSDR may be requested to give its professional opinion on the threats to public health and render its judgment on any proposed remediation level, using data from the prior studies.
ATSDR does not approve or reject the plan but renders an independent opinion on the possible threats to human health. Note that EPA and TDEC have already given their approval on environmental health and levels that are safe for the environment are usually lower than levels that are safe for humans. The stakeholders have opportunity to comment on the ATSDR methodology and results.
If anyone, usually the stakeholders, wishes a higher remediation to be considered by ATSDR or wishes to protest the level as too high there is ample opportunity to do so.

Given the above procedure, it is not surprising that the adopted level is not as high as ATSDR might determine is safe had it been submitted to them. This is unfortunate because it leads to a lot of public misunderstanding.

Discussion

The ATSDR opinions are advisory but only under unusual conditions would another agency go against their professional determination of a significant public health threat. Many persons are surprised to learn that ATSDR has a significant say in matters of public (human) health including appropriate remediation levels and it is very important to understand exactly how these levels are reached. ATSDR cannot suggest the level but is constrained by law and regulation to pass judgment on levels submitted by DOE, EPA, TDEC and the stakeholders, collectively or individually. For these reasons, the level that is finally adopted is less than or equal to the value which ATSDR might approve if it were submitted. Therefore it is NOT correct to conclude that any soil level just above the adopted remediation is a great hazard to health.

Frequently, the ATSDR Public Health Assessment will contain sufficient information to calculate the safe environmental level (including the safety factors inherent in the comparison value and exposure parameters) by calculating the environmental contaminant level that causes an exposure just equal to the comparison value. Also the ratio of the comparison value to the actual dose (often given by ATSDR) can be used to estimate the maximum safe environmental level.

It should be noted that ATSDR was not informed that the earlier (180 ppm) LEFPC, cored samples had been mixed and therefore they made no adjustments in the model. (See Sampling for further comments.) The effect could have varied depending on the model change adopted.

So What is the 400 ppm Level?

The 400 ppm level is the last and highest mercury soil level which was submitted to ATSDR. They rendered the decision that it was safe for public health purposes so it became the approved remediation goal. It is not known with certainty that ATSDR would have approved one of the higher levels being bandied about but there is some informal indication that they would. Due to the lack of crucial information, several conservative assumptions had to be made in computing both the comparison value and the exposure dose. For these two reasons the remediation level of 400 ppm is undoubtedly conservative by a significant but unknown amount. To prove this and answer other questions would probably have cost more than the required excavation. For all practical purposes the participants compromised.

In Conclusion

Risk and exposure calculations are not made with the high degree of precision and confidence of some other physical calculations, e.g., planet orbits, GPS locations, etc. However they are the best estimates we can now produce and their nature should be understood by the public. Above all, they are conservative and not sharp demarcations between safe and unsafe environmental levels.