Look up almost any polymer on the TSCA Inventory and its record carries a two-letter flag: XU. EPA's own glossary explains it in one line. XU "indicates a substance exempt from reporting under the Chemical Data Reporting Rule, (40 CFR 711)."
Every published explanation of XU we could find repeats that sentence and stops. It is accurate and it is not much use, because it answers a question nobody has. The questions people actually have are whether the flag relieves them of anything, whether it survives, and what it says about the substance itself. The answers are in 40 CFR 711, and two of them are not what the one-line definition suggests.
XU appears on 20,333 substances in the July 2026 non-confidential Inventory, making it the most common flag EPA publishes.
What Chemical Data Reporting is, and who it binds
Chemical Data Reporting is a census of what is actually being made in the United States, run every four years. Section 711.8(a) sets the trigger: anyone who manufactured or imported 25,000 pounds or more of a reportable substance at a single site, in any calendar year since the last principal reporting year, has to report on it.
Two adjustments to that number matter. Under 711.8(b) the threshold drops to 2,500 pounds for any substance that is the subject of a rule under TSCA section 5(a)(2), 5(b)(4) or 6, an order under section 4, 5(e) or 5(f), or relief granted in a civil action under sections 5 or 7. Under 711.9 small manufacturers and small governments are excluded entirely, except for substances under those same kinds of TSCA action.
The 2024 submission period ran from June 1 to November 22, 2024. Section 711.20 then puts the recurring window at June 1 to September 30 on a four-year cycle beginning in 2028. That is the date to put in a calendar.
The flag describes the substance, not you
The first thing the one-line definition hides is that these are two different objects. CDR obligations attach to a person, at a site, above a volume. The XU flag attaches to a chemical substance. An exemption on one does not travel to the other.
A company with XU-flagged substances across its whole catalog still has CDR obligations for any other substance it makes above threshold. A company with a single XU substance has not been relieved of anything except reporting on that substance. Reading XU as a statement about your company rather than about a specific chemical is the most common way to get this wrong.
Full exemptions and partial ones are different things
Section 711.6 sets out two kinds of relief, and conflating them produces a real reporting failure.
Full exemptions, at 711.6(a), take a substance out of the rule. The categories include polymers, identified either by the word fragments "*polym," "*alkyd" and "*oxylated" in the Chemical Abstracts Index Name or by named class such as enzyme, lignin, polysaccharide, protein, rubber, siloxane and silsesquioxane. They also include microorganisms and naturally occurring substances.
Partial exemptions, at 711.6(b), do something much narrower. For petroleum process streams and a specific list of other substances, the information described in 711.15(b)(4), which is the processing and use information, need not be reported. The rule says plainly that such substances "are not excluded from the other reporting requirements under this part." You still report. You report less.
XU marks the first kind. We checked that rather than assuming it: all 746 substances the CFR lists by CAS number under the partial exemptions at 711.6(b) appear in our copy of the Inventory, and only 8 of them carry XU. Whatever XU is, it is not the partial-exemption list.
What is actually in the XU set
The full-exemption categories are visible in the data if you look at the 20,333 XU substances together.
17,143 of them, about 84 percent, have a Chemical Abstracts Index Name matching the polymer word fragments in 711.6(a)(1)(i). That single subparagraph accounts for most of the flag. Another 339 substances carry polymer-style names without the flag, which is a useful reminder that the name is a strong hint and the flag is the record.
The remaining 3,190 are mostly the other categories, and reading a few of them makes the structure obvious. Chitin and cellulose triacetate are the named polysaccharide classes in 711.6(a)(1)(ii). Natural gas and natural gasoline are naturally occurring substances under 711.6(a)(3). Dextran hydrogen sulfate sits in the same family.
So XU is, in practice, mostly a polymer marker. That is worth knowing, because it means the flag correlates with a structural class rather than with anything about hazard or commercial status.
The exemption has a condition on it
This is the second thing the one-line definition hides, and it is the part worth checking before relying on the flag.
The opening paragraph of 711.6 withdraws the exemption. A substance described in 711.6(a)(1), (a)(2), (a)(4) or (b) is "not exempted from any of the reporting requirements of this part" if it is the subject of a rule proposed or promulgated under TSCA sections 4, 5(a)(2), 5(b)(4) or 6, the subject of an enforceable consent agreement developed under 40 CFR part 790, the subject of an order under sections 4, 5(e) or 5(f), or the subject of relief granted in a civil action under sections 5 or 7.
Some of those conditions are published on the Inventory as flags of their own: S and SP for significant new use rules, 5E and 5F for section 5 orders, T and TP for section 4 test rules, R for section 6 risk management rules. We checked whether any substance carries XU alongside one of them, and none does. Across all 70,774 substances in the July 2026 release, the count is zero. EPA is keeping the flags consistent with the rule, which means an XU flag is not being silently contradicted by an action flag sitting next to it.
That is reassuring as far as it goes, and it does not go all the way. Two of the conditions in that paragraph carry no Inventory flag at all. An enforceable consent agreement under part 790 is not marked, and neither is relief granted in a civil action under TSCA section 5 or 7. A substance can therefore be flagged XU on the Inventory and still be outside the exemption, with nothing in the published record to show it. The flag is good evidence and it is not proof.
What XU does not tell you
Because it is the most common flag on the Inventory, XU gets read as a general reassurance. It is not one. XU says nothing about whether a substance is safe, nothing about whether it is designated active or inactive, nothing about whether a significant new use rule applies, and nothing about obligations under any other part of TSCA. It is a statement about one census, and only about that census.
In practice, if you are working out whether you owe EPA a Chemical Data Reporting submission in 2028, XU is directly relevant. If you are checking almost anything else, it is not the field you want.
How to check a substance
EPA publishes the non-confidential Inventory as a CSV roughly every six months, with the flags in each record. Two cautions apply to reading it.
The flags arrive from EPA as a single field with multiple values separated by a semicolon, so a substance with two of them appears as text like "PMN; XU" rather than as two separate flags. Any tool that stores that string whole cannot count or filter by flag, and will not find this substance when you search for XU. Ours had exactly that defect until this week.
The second caution is the one that applies to every commercial service built on this data, including ours. EPA maintains a confidential portion of the Inventory that it does not publish. A substance absent from the public file is not thereby absent from the Inventory, and its flags are not knowable from public data at all. Ambrose holds the July 2026 release, 70,774 substances with their flags split and searchable, and every lookup in our regulatory compliance platform reports which EPA release the answer came from.