“There is nothing that this age, from whatever standpoint we survey it, needs more, physically, intellectually, and morally, than thorough ventilation.”  — John Ruskin

I worked on a project once that required me to spend a lot of time in the control building. It was a substantial building with a kitchen and breakroom, several offices, and large restrooms for men and for women. One day, while washing my hands, the ghastliest sounds and smells erupted from one of the stalls. The person standing at the sink next to me complained loudly. “That’s disgusting.”

All I could say was “If not here, where?” But as I looked fruitlessly for a switch to turn on the exhaust fan, what I thought was, If ever there was a time for adequate ventilation, it’s now.

When process safety regulations and standards address adequate ventilation, they’re not talking about exhaust fans in restrooms. What are they talking about? And what is adequate ventilation?

Process Safety Information

In the section on process safety information in the Process Safety Management (PSM) Standard, 29 CFR 1910.119(d), OSHA requires that covered processes include information about ventilation system design. Unlike relief systems, the standard doesn’t also include a requirement for information about the design basis. However, it is a good idea to know how much ventilation you need for process safety when designing a ventilation system. Otherwise, how will you know if it’s good enough?

The amount of ventilation needed depends on what the process needs it for. Some of these are specific to the process, and any competent engineer should be able to calculate them. Some, however, are driven by various codes and standards, what the PSM Standard refers to as RAGAGEP – Recognized and Generally Accepted Good Engineering Practice.

Hazardous (Classified) Locations

Locations where flammable mixtures of air and flammable gases or vapors may form are classified as Class I, Division 1. Locations where those flammable mixtures may only form under abnormal conditions (or where Class I, Division 1 locations need a transition zone) are classified as Class I, Division 2. Beyond that, locations are unclassified. Electrical equipment that goes into Class I, Division 1 locations is expensive to install and maintain. There are more options, less expensive options for electrical equipment that goes into Class I, Division 2 locations. The electrical equipment for unclassified locations is the least expensive of all because it can be ordinary electrical equipment.

As you might imagine, a lot of effort goes into designing a facility to avoid having hazardous (classified) locations.

One RAGAGEP can be found in NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installation in Chemical Process Areas. In §5.5.1., NFPA 497 states that locations with “adequate ventilation, where combustible materials are contained within suitable, well-maintained, closed piping systems” can be unclassified locations. To make clear what it means by “suitable, well-maintained, closed piping systems,” NFPA 597 goes on to state that locations “that lack adequate ventilation, but where piping systems are without valves, fittings, flanges, and similar accessories that may be prone to leak” can also be unclassified locations.

So, if the ventilation is not adequate, piping systems must be all-welded, or the area around them must be classified.

In this case, then, what is adequate ventilation?

In §5.5.2., NFPA 497 goes on to describe “adequate ventilation.” It includes being outdoors, being in a building that is “substantially open and free of obstruction” (like a park pavilion), or space “provided with ventilation equivalent to natural ventilation”. So, what is “equivalent to natural ventilation”? NFPA 497 doesn’t define “natural ventilation”, but going back to the general definitions, §3.3.1 gives an explicit quantitative definition. Adequate ventilation is “a ventilation rate that affords six air changes per hour, 1 cfm per square foot of floor area, or other similar criterion that prevents the accumulation of significant quantities of vapor/air concentrations from exceeding 25 percent of the lower flammable limit (LFL).”

(Note: For those that are curious but don’t want to do the math, six air changes per hour equals 1 cfm per square foot of floor area when the height of the space is 10 feet. Under 10 feet, six air changes per hour is less, while above 10 feet 1 cfm per square foot of floor area is less.)

Laboratory Hoods

There are several standards for laboratory hoods and most rely on face velocity as the measure of adequate ventilation. (OSHA has a standard that addresses laboratory hoods, 29 CFR 1910.1450, Occupational exposure to hazardous chemicals in laboratories, but it does not quantify “adequate ventilation.”) The standards most commonly referred to in the United States are ANSI/ASSP Z9.5-2022, Laboratory Ventilation, and ANSI/ASHRAE 110-2016, Methods of Testing Performance of Laboratory Fume Hoods.

The standard from the American Society of Safety Professionals repeats the point made in early version of Z9.5 published by the American Industrial Hygiene Association that no face velocity is universally acceptable. However, there is general agreement that an average face velocity of a hood is sufficient to capture and contain hazardous chemical vapors released in the hood when it is in the range of 80 to 100 fpm, and that velocities above 150 fpm are likely to introduce so much turbulence as to be less effective.

Obviously, the face velocity is dependent on how much the fume hood sash is opened, so a height must be stipulated. Commonly that is around 18”.

Fume Exhaust

It is not uncommon for production areas that use flammable gases or vapors to use local exhaust to vent the gas or vapor. For instance, polyethylene foam is manufactured with isobutane as the foaming agent. After the foam expands, the process vents the isobutane. Another example is the elephant trunk that many facilities use to vent vapors released while filling drums.

In these cases, there are two concerns. One is the face velocity necessary to pick up the vapors. For this, the discussion about laboratory hoods is appropriate. The other is about assuring that the flammable vapor is sufficiently diluted to prevent forming a flammable mixture with air that could ignite within the duct.

NFPA 86, Standard for Ovens and Furnaces, sets the maximum concentration for an exhausted air/vapor mixture at 25% of the LFL if the exhaust is not monitored, and 50% of the LFL if it is monitored. When calculating whether ventilation is adequate, it is important to keep in mind that flammable limits are expressed as volume-per-volume percentages, and that volume is related to molecular weight. The volume of a g-mole of gas is 22.4 liters at standard conditions.

Consider the case of isobutane. It has an LFL of 1.6% (by volume). A mixture of isobutane in air at 25% of the LFL would be 0.4% isobutane (by volume). The molecular weight of isobutane is 58.12 g/g-mol. If the foam extruder was using isobutane at a rate of 100 g/min, that would be 1.72 g-mol/min, or 38.54 liters/min, which converts to 1.36 scfm. To stay below a maximum concentration of 0.4% isobutane (by volume) would require a total flow rate of 340 scfm, which would be adequate ventilation.

When Process Safety Depends on Adequate Ventilation

There are many circumstances in a process facility where ventilation is not just about personal comfort, but is a significant component of process safety. When it is, it is essential to know what, exactly, constitutes “adequate ventilation.” It’s not a matter of opinion, but something that can and should be quantified. There are recognized and generally accepted good engineering practices to guide that effort. Make sure you take advantage of them, and if you have a PSM-covered process, make sure you have documented them as part of your process safety information.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.