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Science Terms Definitions - O


A substance that causes tumors, whether benign or malignant.

One-hit Model

The basic dose-response model based on the concept that a tumor can be induced by a single receptor that has been exposed to a single quantum or effective dose unit of a chemical.

Operable Unit

Term for each of a number of separate activities undertaken as part of a Superfund site cleanup. A typical operable unit would be removal of drums and tanks from the surface of a site.

Operation And Maintenance

Activities conducted after a Superfund site action is completed to ensure that the action is effective. Actions taken after construction to assure that facilities constructed to treat waste water will be properly operated and maintained to achieve normative efficiency levels and prescribed effluent limitations in an optimum manner. On-going asbestos management plan in a school or other public building, including regular inspections, various methods of maintaining asbestos in place, and removal when necessary.

Oral Toxicity

Ability of a pesticide to cause injury when ingested.


Occupational Safety and Heath Administration of the U.S. Department of Labor. Federal agency with safety and health regulatory and enforcement authorities for most U.S. industry and business.


The place where effluent is discharged into receiving waters.

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