## Facts

- Ethylene oxide (EtO) is a gas used in manufacturing and hospital instrument sterilization; OSHA found it carcinogenic and associated with serious health risks.
- OSHA conducted rulemaking to regulate occupational exposure to EtO using epidemiological, animal, and other scientific evidence.
- OSHA’s risk assessment assumed a proportional dose-response relationship and treated EtO as having no safe exposure threshold.
- In 1984 OSHA issued a final standard setting a permissible exposure limit (PEL) of 1 ppm as an 8-hour time-weighted average and requiring additional measures at 0.5 ppm.
- OSHA’s proposal had included a short-term exposure limit (STEL) (e.g., 10 ppm over 15 minutes), but the final rule omitted any STEL.
- Public-interest petitioners challenged the standard as insufficiently protective (including the omission of a STEL) and raised objections to executive-branch involvement in the rulemaking process.
- Industry petitioners challenged the stringency and scientific basis of the 1 ppm PEL, including OSHA’s no-threshold assumption and low-dose extrapolation methods.

## Issues

1. Whether substantial evidence supported OSHA’s determination of significant risk and its selection of a 1 ppm 8-hour TWA PEL for EtO.
2. Whether OSHA’s decision to omit a STEL was supported by the record and accompanied by an adequate explanation.
3. Whether objections concerning executive-branch involvement in the rulemaking required judicial resolution in this posture.

## Decision

- The court affirmed OSHA’s 1 ppm 8-hour TWA PEL as supported by substantial evidence.
- The court held OSHA’s omission of a STEL was not supported by the record and lacked an adequate reasoned explanation.
- The court remanded the rule to OSHA for further consideration of a STEL (or a reasoned justification for not adopting one).
- Because of the remand, the court declined to reach the broader constitutional and statutory objections concerning executive-branch involvement.

## Legal Principles

- OSHA standards must be supported by substantial evidence and reflect reasoned decisionmaking connecting the record to the regulatory choices made.
- OSHA may regulate based on the best available evidence despite scientific uncertainty and may use conservative risk-assessment assumptions if rational and record-supported.
- When an agency materially changes course between a proposal and a final rule, it must explain the change in a manner supported by the administrative record.
- A reviewing court may decline to decide constitutional or structural challenges where remand on nonconstitutional grounds is sufficient to resolve the petition.

## Conclusion

The D.C. Circuit sustained OSHA’s long-term EtO exposure limit based on a record-supported risk assessment, but required reconsideration of short-term exposure controls because OSHA did not adequately justify abandoning a proposed STEL despite record evidence indicating potential risks from exposure spikes.


Public Citizen Health Research Grp. v. Tyson, 796 F.2d 1479 (D.C. Cir. 1986)