Applicability of HMR to Naturally Occurring Microorganisms
Pipeline and Hazardous Materials Safety Administration · Jul 16, 1992 · Reference No. 92-0102
Interprets: 49 CFR § 173.134
Reggie’s quick take
Someone asked PHMSA to exempt all naturally occurring microorganisms that are not virulent pathogens from regulation as infectious substances under the Hazardous Materials Regulations. The agency said such a microorganism would not be regulated under the HMR if it is not deemed dangerous and does not meet the definition of an infectious substance in § 173.134. That definition covers viable microorganisms or toxins that cause or may cause disease in humans or animals, including agents listed in 42 CFR 72.3 or any agent that causes or may cause severe, disabling or fatal disease.
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
This is in response to your letter and telephone conversation with Ms. Eileen Edmonson Martin of my staff requesting that all naturally occurring microorganisms not classified as virulent pathogens be exempt from regulation as an infectious substance (etiologic agent) under the Hazardous Material Regulations (HMR; 49 CFR Parts 171-180). Specifically, you state that these naturally occurring microorganisms "have not been deemed to be dangerous."
Section 173.134 defines an infectious substance as "a viable microorganism or its toxin, which causes or may cause disease in humans or animals, an includes those agents listed in 42 CFR 72.3 of the regulations of the Department of Health and Human services or any other agent that causes or may cause severe, disabling or fatal disease." If as you state, the naturally occurring microorganism is not deemed to be dangerous and does not meet the definition for an infectious substance, then it would not be regulated under the HMR.
I appreciate your inquiry. Please feel free to contact us if you have additional questions.
A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.