After a chemical release everyone asks the same first question: what was it. The second question decides more: how much of it reached a person, and for how long. Toxicology has a rule for that, the Texas Supreme Court has quoted the rule, and almost nothing a person notices at the scene answers it.
The rule. “One of toxicology’s central tenets is that ‘the dose makes the poison’” (Borg-Warner Corp. v. Flores). The federal agency for toxic substances defines the word. Dose is “The amount of a substance to which a person is exposed over some time period,” and “In general, the greater the dose, the greater the likelihood of an effect.” The agency separates two things that people run together: “An ‘exposure dose’ is how much of a substance is encountered in the environment,” and “An ‘absorbed dose’ is the amount of a substance that actually got into the body through the eyes, skin, stomach, intestines, or lungs” (ATSDR). For something in the air, the first is built from two numbers a person does not have at the time: the concentration, which is how much of the substance was in the air where he stood, and the minutes he stood there. The second depends on the body as well.
The yardsticks. For emergencies there is a published set of yardsticks, the acute exposure guideline levels. They “represent threshold exposure limits” “for the general public and are applicable to emergency exposures ranging from 10 min to 8 h.” There are three. Above the first, people “could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects,” effects that “are not disabling and are transient and reversible upon cessation of exposure.” Above the second, “irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.” Above the third, “life-threatening adverse health effects or death.” And each level is set five times over, because the levels “are developed for each of five exposure periods (10 min, 30 min, 1 h, 4 h, and 8 h).” That is the rule of dose built into the yardstick itself: air a person could stand for ten minutes is a different matter over eight hours. Two cautions are written into the definitions. The levels are set for “the general population, including susceptible individuals,” and the committee still says that people “subject to unique or idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL” (National Research Council). And every one of them is a concentration of a named chemical in the air, measured or estimated. None of them is the incident level announced in a public alert, which describes the event and measures no one’s air.
What the nose reports. Smell is not one of the measurements. The same committee describes the air below its first level as air that can produce “mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation”: a person can sometimes smell a thing before it reaches the first yardstick. The reverse is true of other gases, which do their harm below what a person can smell; of one of them the federal medical guideline says, in the words Hours Later quotes, that “odor provides insufficient warning of hazardous concentrations.” The Quiet Gas is about a gas with no smell at all, and Odorant is about a smell that had to be added to a gas on purpose. So a strong smell is not proof of a harmful dose, and no smell is not proof of a safe one. Whether an odor is harmless is a statement about concentration, and concentration is measured, not smelled.
What a building does to it. Going indoors changes the dose without ending it. The air inside a closed house lags the air outside: the peak is lower, and it comes later and leaves later. The minutes that count therefore include the minutes after the cloud has gone, if the house stays shut, which is the science behind Shelter in Place and the reason the all-clear matters as much as the order.
What the body reports, and when. A quiet evening is not a measurement either. Some gases burn the moment they are breathed, and the pain sends a person out of the cloud. Others do almost nothing for hours and then fill the lungs with fluid, which is the whole subject of Hours Later. So the hour at which a symptom began is itself evidence of what the exposure was, and an examination that night or the next morning records what an examination a month later cannot.
What the law asks. The Texas Supreme Court applied the tenet in a case about asbestos: a mechanic who ground brake pads, in the Court’s words, “in a room that measured roughly eight by ten feet.” The Court held: “In a case like this, proof of mere frequency, regularity, and proximity is necessary but not sufficient, as it provides none of the quantitative information necessary to support causation under Texas law.” It also said what would do: “Defendant-specific evidence relating to the approximate dose to which the plaintiff was exposed, coupled with evidence that the dose was a substantial factor in causing the asbestos-related disease, will suffice.” And it said that causation “need not be reduced to mathematical precision” (Borg-Warner Corp. v. Flores). That case was about years of dust, not an evening of vapor, and the Court was writing about asbestos. But the question it asks is a useful first question in any exposure case: how much. Approximate is the word to hold onto, but it is half of the test. The law does not ask for a number to the decimal. It asks for evidence from which the amount can honestly be estimated, and then for evidence that an amount of that size was a substantial factor in causing the illness. The mechanic in that case lost. The Court rendered judgment for the defendant, because the record, it said, “reveals nothing about how much asbestos” he might have inhaled.
Where the dose is written down. In four places, and none of them is memory. The first is the plant’s own reports. A Texas plant with a reportable release must tell the state, within 24 hours, “the estimated duration of the emissions” and “the estimated total quantities” released (30 Texas Administrative Code Section 101.201). The federal emergency notice must carry “An estimate of the quantity of any such substance that was released into the environment” and “The time and duration of the release” (42 U.S.C. 11004). A plant covered by the federal Risk Management Program rule must record the “Chemical(s) released,” the “Estimated quantity released in pounds” and the “Weather conditions, if known” (40 CFR 68.42). Those say how much left the plant. They do not say how much reached a porch. The second is the air monitors, and a reading is a concentration at one place at one minute: what matters is who took it, where, when, and what the instrument could and could not detect. The third is the person’s own account, fixed while it is fresh: where he was, for how long, indoors or out, with the windows open or shut and the air conditioning on or off. The fourth is the body, in a medical record with a time on it. The first two are in other hands, and they do not keep themselves (Spoliation).
Why it matters. What was it is the question for the first hour. How much, and for how long, is the question for everything after. The answer is almost never what it smelled like, or how anyone felt on the drive home.
Borg-Warner Corp. v. Flores, No. 05-0189 (Tex. June 8, 2007), as posted on FindLaw; Agency for Toxic Substances and Disease Registry, Public Health Assessment Guidance Manual, Glossary (the entries for dose and concentration); National Research Council, Acute Exposure Guideline Levels for Selected Airborne Chemicals, Volume 4 (National Academies Press, 2004, as hosted by the Environmental Protection Agency; the definitions in its introduction); 30 Tex. Admin. Code § 101.201, 42 U.S.C. § 11004 (subsection (b)) and 40 CFR § 68.42 (paragraph (b)), as posted by Cornell LII. The sentence about odor and hazardous concentrations is from the federal medical management guideline quoted and linked in Hours Later. This page gives no exposure number for any chemical on purpose: the numbers differ by chemical and by length of exposure, and they are for the people who measure. This is general information, not medical or legal advice. Text and figures as published at publication.
