What are air exchanges? Should USP <800> require a specific number of them?

Billboard that asks "Should USP <800> specify air exchange rates?This is an area where an industrial hygienist with strong skills in ventilation requirements analysis could be very useful to compounding pharmacies in terms of energy savings and protection from airborne hazards over the long term. In that spirit, we offer the following discussion.

A room contains a volume of air; replacement of all the air in a room with ‘new’ air – also called replacement air — is one air exchange.

But an air exchange doesn’t happen all at once, so the math for applying an air exchange rate to the design of a ventilation system can be tricky.

So recommended air exchanges are generally expressed as a range, not a specific number.

It makes sense that air exchanges can help determine the comfort level and acceptable air quality in a room.

Dilution ventilation and local ventilation

Air exchanges are a form of dilution ventilation, in which new, cleaner air is blown into a room  while dirtier air is exhausted out of the room.

Lab hoods are a form of local ventilation, in which contaminants are pulled away from workers’ breathing zones as close to the contaminants’ source as possible.

Both types of ventilation, dilution and local, are used in industrial and laboratory situations.

A Washington State Dept. of Labor & Industries website published the useful chart below to compare the two types of ventilation.

Table comparison of dilution and local ventilation strengths and weaknesses

As you can see, the chart addresses many areas of concern, including volume of toxic substances, degree of toxicity, maintenance requirements and energy costs.

Compounding pharmacies are not identical

Variety among compounding pharmacies includes number of employees, size of building, number of rooms, ceiling heights — all can greatly influence ventilation requirements.

Compounding pharmacies need both local and dilution ventilation, but both should be right-sized for the tasks people perform in the pharmacy.

Air exchange “standards”

It appears there are no mandatory air exchange rates for labs or compounding pharmacies other than suggested ranges for comfort.

OSHA recommends the guidance in Prudent Practices in the Laboratory: Handling and Management of Chemical Hazards, a book available from the National Academies Press.

You can come across reference OSHA non-mandated ‘recommendation’ of 8-12 air exchanges per hour for laboratories on many websites. However, it is difficult to trace the source of that recommendation.

Industrial hygienists

In their much used, often cited industrial ventilation manual, the American Conference of Governmental Industrial Hygienists says,

Air changes per hour” or “air changes per minute” is a poor basis for ventilation criteria where environmental control of hazards, heat, and/or odors is required.

“The required ventilation depends on the problem, not on the size of the room in which it occurs.”

A summary

A numerically specific air exchange rate is difficult to engineer into a ventilation system.

When compared to local ventilation, dilution ventilation (air exchanges) is not the most effective method for removing highly toxic airborne hazards from workplace air.

High air exchange rates produce high energy costs.

Are there additional ways to approach the ventilation of compounding pharmacies?

Given the lack of mandatory requirements, and the opinion of experts like industrial hygienists, what should the USP recommend?

Does specification of a specific number of air exchanges per room provide adequate or improved employee safety?

Should the USP provide a list of criteria that determine air quality and airborne hazards in each pharmacy? (In addition to air pressure and air exchanges, such criteria might include airborne particle counts.)

If a criteria list existed, would pharmacists have access to the tools they need to meet it?

We think compounding pharmacists may want to talk about the air-exchange issue.

Additional USP 800 comments here and here.

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If you want to talk about non-sterile HD compounding, call Sentry Air Systems at 800.799.4609. You can also email us at sales@sentryair.com, visit our website or fill out the feedback form below.



Industrial Ventilation

Industrial Ventilation, a Manual of Recommended Practice, American Conference of Governmental Industrial Hygienists

Chapter16. Laboratories, ASHRAE handbook on laboratories

University of North Carolina UNC Laboratory Design Guidelines