Which term is used for the amount of material in the air expressed at a ratio compared to the volume of air?

The term that describes the amount of material in the air expressed as a ratio compared to the volume of air is indeed "Parts Per Million" (PPM) and "Parts Per Billion" (PPB). These units allow for the quantification of very small concentrations of hazardous substances within a larger volume of air.

Using these measurements, one can accurately assess the degree to which a hazardous material is present in the environment, which is crucial for evaluating potential exposure risks and implementing safety measures. For example, knowing the concentration of a toxic gas in terms of PPM can help determine the necessary protective measures or responses needed during a HAZMAT incident.

The other terms mentioned refer to different concepts: Permissible Exposure Limit (PEL) relates to the maximum allowable concentration of a substance in the workplace air, and while it is related to exposure, it does not specifically express the concentration ratio itself. Radiation Absorbed Dose relates to the amount of radiation energy absorbed by tissue and is not applicable to airborne contaminants. Lethal Dose (LD50) refers to the amount of a substance that is sufficient to cause death in 50% of a population, which is also not a measure of concentration in the air.