If you read this blog you can tell I'm a curious kind. I want to know why things work the way they do. Why, for example are there five categories of waste numbers? If there is a "D", "F", "K", "P", and "U" was there ever an "A", "B", or "C:" ?
I asked this question a long time ago to the EPA and they told me that it was also a question that some of them had asked but apparently been lost somewhere in the history of what took place between RCRA the law and RCRA the regulation.
So while I was searching for how the Toxicity Characteristic thresholds - D004 - D043 - were established, I came across an EPA document with the very friendly title:
Identification and Listing of Hazardous Waste 40 CFR§261.4(b):
Exclusions: Solid Wastes which are Not Hazardous Wastes.
A User- Friendly Reference Document
Version 1: October 2009
And in this document it linked to two Federal Registers:
Proposed Rule- Hazardous Waste Guidelines and Regulations. 43 FR 58946, December 18, 1978
Final Rule, Interim Final Rule and Request for Comments - Hazardous Waste Management System: dentification and Listing of Hazardous Waste. 45 FR 33084, 33120, May 19, 1980Interesting, been in the hazardous waste business since 1984 and never read either of these two documents. The cool thing - if you think historical context is cool - is that the proposed and final rule Federal Registers give the thinking and reasoning behind the actual regulation being established.
Maybe, I thought, they might shed light on any other letters used for establishing the waste numbers. Were there other "lists" established in the beginning before the final rule in 1980?
Well I downloaded them, read them, and, darn, no luck. Nothing about any "A" list or "M" list. On the other hand, it did answer some other questions that I have been pondering lately. That is, what does the number 5 mg/L actually mean?
If you read my last bazzilion posts on arsenic in apple juice you can see that one of my driving points is that you cannot compare a quantity in one thing with a regulatory threshold established for another thing. The 10 ug/L arsenic MCL is for drinking water, the amount of arsenic found in apple juice cannot be compared to that number because of how the 10 ug/L was established. 10 ug/L of arsenic MCL is the "safe" level based on drinking 2 liters of water for 70 years. What arsenic is found in apple juice must be looked at based on how much apple juice is consumed. That's why the FDA established a safe level of arsenic in apple juice of 23 µg/L. Quite simply, you drink less apple juice than water so you can consume more arsenic per unit of liquid.
That's a difficult concept to get across to folks. But it all comes back to the basic toxicological assumption that there is a dose that will not pose harm. That dose is based on the concept of unit of substance per unit of body weight per day. We call this the reference dose and all our thresholds are supposed to be based on this.
How much, in theory, will the receptor receive - uptake - based on their body weight? The idea here is that there is an amount per kilogram of body weight that we can assume will pose no adverse health concern over a period of time. For the public it is 70 years for a chronic health concern, for an employee it is 30 years of exposure based on a 5 day, 40 hour work week.
What we do is draw a line in the sand - a threshold - that says based on these conditions (time exposed, uptake expected) exposure to below this amount is considered "safe" for the population as a whole. Now we are very conservative in this approach. We look at worst-case situations such as little Johnny jumping the fence and placing 40% of his body into the creek water five times a week as well as our uncertainty with the data we have and how what we see in animals might relate to us humans.
So the threshold we report as "safe" is in most cases very-very-very safe. It's not a perfect science, but it is the best we have and because "people demand a number" it allows us to establish a limit - a line in the sand.
Numbers mean something. The context behind them is why they have value. Unfortunately we lose sight of the why we drew the line there and instead focus on the number as if it is sacrosanct.
And so it goes with RCRA as well. The arsenic threshold between a RCRA hazardous waste D004 and a non-RCRA hazardous waste is 5 mg/L. What does 5 mg/L mean, other than you got yourself a hazardous waste? Why 5 mg/L? What's it based on?
Well I found out the answer to that question.
Next post: RCRA...in the beginning. Part 2: EP Toxicity