“The guard shall be such that it does not offer an accident hazard in itself.”  — 29 CFR 1910.212(a)(2) Machine guarding

It’s hard to imagine any safeguard that doesn’t pose its own burden. Safety shoes are heavy and make a worker less nimble. Side shields on safety glasses impair peripheral vision. Interlocks introduce the prospect of spurious trips. A double-block-and-bleed valve configuration creates a dead leg where contaminants can accumulate and triples the number of components with the potential for failing. And people still complain about the possibility of someone wearing a seatbelt being trapped in a burning car or drowning in a submerged car.

Every safeguard is a tradeoff between advantages and disadvantages, OSHA’s regulation on machine guarding notwithstanding. A tradeoff, but not an even tradeoff. The advantages of the safeguard far outweigh the disadvantages. And with time, we learn to compensate for the disadvantages while enhancing the advantages.

The same holds true for exhaust ducts, which are an important safeguard in workspaces. The advantages far outweigh the disadvantages, but still, there things we can do to reduce the hazards that come with exhaust ducts.

Why Have Exhaust Ducts?

If you didn’t know, the purpose of exhaust ducts is to remove harmful or undesirable air from the space being exhausted. With a heating, ventilation, and air conditioning (HVAC) system, the air is undesirable because it is the wrong temperature or because it has the wrong humidity. The exhaust duct actually returns the air to the HVAC unit, which treats it to make it desirable again and then discharges it back to the space. However, HVAC systems cannot remove smoke. Smoke in an HVAC system is bad. So, NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilation Systems requires that ducts in HVAC systems carrying more than 2,000 cfm of air be equipped with duct smoke detectors that alarm, close the dampers, and shut down blowers and fans. No one argues that pulling smoke from a fire through an HVAC is a good idea.

In process areas, exhaust ducts prevent the accumulation of dangerous concentrations of flammable gases and vapors, or prevent the accumulation of dangerous concentrations of toxic gases and vapors. Once the exhaust ducts remove the flammable or toxic gases and vapors from the workspace, they must discharge them somewhere else. In some cases, when the amount is small, the duct discharges them to the atmosphere. When the amount is larger, discharge directly to the atmosphere is unacceptable. In those cases, flammable gases and vapors are burned in a thermal oxidizer, while toxic gases and vapors are converted to something less hazardous in a scrubber.

Toxic Gas Exhaust Systems

The exhaust system for removing toxic gases must reduce the concentration in the workspace to a safe level. For those toxic gases that OSHA has regulated, the safe level is the Permissible Exposure Limit (PEL), which is specific to each toxic material. If there is no PEL, then we must rely on the Recommended Exposure Limit (REL) developed by the NIOSH. The ventilation system is adequately sized if it can keep the concentration in the workspace below the PEL or REL.

Natural draft may be adequate if no air pollution control measures are required, but natural draft is not going to be adequate if they are. That means blowers or fans. Ideally, the duct will be under an induced draft, so that most of the duct is under a slight vacuum. A leak in the duct is going to pull air in, rather than force toxic gases out.

It’s important to keep in mind that the PEL of most toxic gases and vapors is very low – typically in the part-per-million range.  It probably comes as no surprise that the PEL of cyanide gas is low: 10 ppm. For ethylene oxide, the PEL is 1 ppm; for hydrogen chloride, it’s 5 ppm; and for ammonia, 50 ppm.

It’s also important to keep in mind that many scrubber solutions are not harmless. When water can be used, that’s great. However, scrubbers often use caustic solutions or sulfuric acid solutions, both of which are very corrosive.

Like HVAC systems, toxic gas exhaust and treatment systems must be monitored. They should be monitored continuously for concentration at the source. The blower should be monitored for operation (continuously is preferred, although the concentration monitor may be able to pick up a blower failure), and the scrubber solution pump should be monitored. When any monitor indicates a failure, it should cause an alarm to prompt an operator response. It should also cause a trip that shuts off the source. Beyond that, the response should be suited to the system and the toxic gas, keeping in mind that protecting workers is the immediate concern and that protecting the environment is also important. Unlike HVAC systems, which recirculate the air, there is probably no good reason to shut the working components of the system down.

Flammable Vapor Exhaust Systems

The most important criteria for an exhaust system that removes flammable vapors is that it does not ignite flammable mixtures of those vapors. That means avoiding flammable mixtures and avoiding ignition sources. Ignition sources in an exhaust duct can include sparks at the inlet to the duct, electrical instruments installed in the duct, the fan or blower, and for those that tie into a thermal oxidizer, the thermal oxidizer itself.

For this reason, the most important requirement of the exhaust system is that it be designed to keep the air/vapor mixture below the lower flammable limit (LFL) of the vapor mixture. How much below? The confined space standard defines a maximum concentration of 10% of the LFL ( 29 CFR 1910.146(b) ). But that is for human occupancy.

In ducts, which are not intended for human occupancy, the limits are higher. NFPA 33, Standard for Spray Application Using Flammable or Combustible Materials, requires that exhaust air that is recirculated not exceed 25% of the LEL and that the concentration be continuously monitored. At any point that the concentration exceeds 25% of the LFL, the air must be diverted to the atmosphere. NFPA 86, Standard for Ovens and Furnaces, sets the maximum concentration for an exhausted air/vapor mixture at 25% of the LFL if the mixture is not monitored, and 50% of the LFL if it is monitored. Even when neither NFPA standard applies to a particular exhaust system, they both give a sense of what NFPA considers a safe margin.

As for sources of ignition, avoid them or eliminate them outright. For electrical components, that means assuring that they have the proper electrical classification. For mechanical components, that means using non-sparking construction.

Thermal Oxidizers

Flashback from a thermal oxidizer is a concern that must be addressed in the design of an exhaust system with this air pollution control measure. Experts agree that the maximum concentration for an exhausted air/vapor mixture must not exceed 25% of the LEL if the mixture is not monitored and must not exceed 50% of the LEL if it is monitored.

What if it does? When the thermal oxidizer is operating, it is an ignition source. A flammable mixture in the duct will be ignited. A flame front will start at the thermal oxidizer and travel back to the source, accelerating as it goes until it reaches the speed of sound. Then the flame front will transition from a deflagration, where the flame front propagates by thermal conductivity, to a detonation, where the flame front propagates by shock compression.

That shock compression can be enough to blow a duct apart and ignite the resulting vapor cloud. A flame arrestor in the exhaust duct near or at the end closest to the initial ignition can be enough to cool and extinguish the flame front before it completes its path down the duct.

IVEX in Sidney, Ohio

In 2013, IVEX Protective Packaging experienced an explosion when the foaming agent in its foam extrusion process, isobutane, ignited. The process exhausted the isobutane to a regenerative thermal oxidizer (RTO), which ignited it, causing a backflash. OSHA cited the company for 21 safety violation and issued a fine of $128,700. The incident injured three workers and caused significant property damage, but fortunately, didn’t kill anyone.

A Trade-Off

Exhaust ducts are an important safeguard for protecting workers from dangerous components in the air they breathe. However, they have their own hazards, typically related to those same dangerous components. It is possible to get the full benefit of exhaust systems, while at the same time minimizing the hazards.  But only with a good understanding of those hazards.

Perhaps it’s time to take another look at exhaust systems in your facility. When they work properly, you should experience all of the advantages. On the other hand, it’s just as important to understand how they can fail. How will you know when they do? And when they do, how should the sy


  • 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.