What Does Chemical Hold-Out Mean?
Chemical hold-out refers to the ability of a piece of protective equipment to prevent hazardous substances from crossing the protective barrier between the space that contains the hazardous substance and the space occupied by a worker.
It is used specifically to reference the protective capacities provided by personal protective equipment (PPE), especially garments such as coveralls. Hold-out ability can refer to chemical substances in solid, liquid, gas, and vapor forms, as well as to the ability to keep out particle emissions from radioactive chemicals.
Safeopedia Explains Chemical Hold-Out
The concept of hold-out is used as a general term in relation to more-specific measures of protective capacity, such as “breakthrough time” and “cumulative permeation.” These measures refer to the ability to limit the rate or total amount of chemical from crossing a protective barrier within a defined time period, such as an hour. Because the protective capacity of any given protective material varies depending on the chemical substance it is exposed to, as well as the chemical state of that substance (e.g. solid, liquid, etc.), a suit's hold-out ability is always relative to the specific chemicals to which it will be exposed.
All major occupational health and safety organizations require hazard assessments to determine what hazardous substances workers are likely to be exposed to, and they impose obligations to provide safety measures such as PPE to limit exposure to safe levels. The hold-out capacity of a given piece of PPE provides employers with an ability to understand the level of exposure that will be experienced by employees wearing that PPE in the field.
When hold-out is used to describe “breakthrough time” (the most common method of describing hold-out ability), the protective capacity being measured is the time it takes for a substance to completely permeate a given protective material. Breakthrough time is the amount of time it takes for the rate at which a hazardous substance can cross the barrier provided by a given protective material to become unacceptably high. For instance, the National Fire Protection Association’s (NFPA) 1991 Barrier Requirements give a rate of 0.1 micrograms per cm2 per minute (µg/cm2/min) as the breakthrough point.
Because transmission across a protective barrier does not count as breakthrough until the breakthrough point has been reached, relevant hold-out data may also include measurements of cumulative permeation for a given period of time (e.g. the first hour of exposure), as well as average permeation per minute during the pre-breakthrough period. Although hold-out is a chemical-specific concept, PPE may be said to have high hold-out in general if it can effectively hold out a wide variety of substances relevant to the industry it is being used in.
Chemical hold-out is used in reference to both protective equipment for everyday use and equipment used by emergency responders such as firefighters. The NFPA prescribes hold-out thresholds for both situations. These minimum thresholds are equipment-specific, and they include garments, gloves, and self-contained breathing apparatuses (SCBAs).