“Suffocation is a cruel way to go.”  — Arlaina Tibensky

I usually think of process safety hazards as being related to fire, explosions, or toxic releases. There is another process hazard, however, doesn’t necessarily depend on any of these. It’s asphyxiation—death due to an inability to get enough oxygen to the body.

Asphyxiation is not a hazard uniquely associated with the process industries. But there are some asphyxiation hazards that deserve special attention when considering process safety.

Causes of Asphyxiation

One of the common causes of asphyxiation is drowning. This happen when people inhale water, which interferes with the ability of air to get into the lungs, where oxygen is absorbed by the blood and carried to the rest of the body.

Choking is another cause of asphyxiation. A foreign object becomes lodged in the airway, plugging it, so preventing air from getting to the lungs. Different from choking, which is internal, strangulation causes asphyxiation by closing off the airway with external pressure applied to the throat.

Two other causes of asphyxiation—anaphylaxis and asthma—cause the upper airways to swell and constrict. Anaphylaxis is a severe allergic reaction to insect venom, some foods, and occasionally, to some medicines. An asthma attack is also the result of upper airways swelling and constricting. Many things can trigger an asthma attack, including respiratory infections, stress, or even the ordinary allergens that cause others to have itchy eyes and a runny nose.

But these causes of asphyxiation have little to do with the process industries. There are three causes that do: engulfment, simple asphyxiants, and chemical asphyxiants.


As a young process engineer working in New England, one of the first cases of asphyxiation by engulfment that I ever heard of involved cranberries. A worker slipped into an open vat of cranberries and in his struggle to get out, sank. It was like quicksand. It was several minutes later before he was retrieved, already dead from asphyxiation.

More often, engulfment hazards are associated with grain silos, but engulfment can occur in any enclosed vessel that contains a liquid or a solid that can flow. Engulfment can include drowning when the liquid is water. Interestingly, engulfment hazards are most often associated with non-toxic materials, particularly foods. It is their non-toxicity that tempts a facility to leave vessels open, creating the opportunity for falling in or going in deliberately, unmindful of the hazard.

Engulfment is so serious a hazard that OSHA identifies it as one of four conditions that is sufficient, by itself, to make a confined space permit-required.

Simple Asphyxiants

Simple asphyxiants are gases. They are not especially toxic—many are non-toxic—but they are not air. They kill by displacing air with something that doesn’t contain oxygen. The most common simple asphyxiant in the process industries is nitrogen used for inerting. Cryogenic nitrogen is also used for flash freezing in some processes. Other simple asphyxiants include the noble gases: helium, argon, neon, krypton, xenon, and arguably, radon. Except for radon, which is radioactive, the noble gases are harmless, except for the fact that they aren’t air.

However, gases don’t need to be inert to be simple asphyxiants. Hydrogen, a very flammable gas, is non-toxic and is classified as a simple asphyxiant. Other flammable gases—methane, ethane, ethyne, acetylene, propane, butane, etc.—are also simple asphyxiants. They are not harmless, but the concentration at which they are considered Immediately Dangerous to Life and Health, their IDLH, is 10% of their lower explosive limit (LEL).

The normal concentration of oxygen in air is 20.9%. OSHA considers a concentration less than 19.5% oxygen to be dangerous, although some set the safe level at 19%. A concentration less than 10% oxygen is fatal. For a mixture of air and a simple asphyxiant to be a fatal asphyxiation hazard, it must contain at least 52% of the simple asphyxiant. Even to be below OSHA’s safe limit, a mixture must contain at least 6.7% (67,000 ppm) of the simple asphyxiant. The LEL of flammable hydrocarbons are in the range of 1% to 5%, so a mixture of air and a flammable gas is a flammability hazard long before it becomes concentrated enough to be an asphyxiation hazard.

The most important safeguard against simple asphyxiants is adequate ventilation. Indoors, especially in small rooms where ventilation is inadequate, a good practice is to install oxygen monitors to alert personnel to oxygen-insufficient atmospheres.

Chemical Asphyxiants

Chemical asphyxiants, also called toxic asphyxiants, don’t simply displace air. They prevent the body from transporting oxygen in the lungs to where it is needed in the body. Most people are aware of the hazards of carbon monoxide, a chemical asphyxiant. There are others.

  • Carbon monoxide, which binds to hemoglobin to block delivery of oxygen, has an IDLH of 1,200 ppm.
  • Hydrogen sulfide, which disrupts respiration in cells, has an IDLH of 100 ppm.
  • Hydrogen cyanide, which disrupts respiration in cells, has an IDLH of 50 ppm.
  • Phosgene, which disrupts the blood-air barrier, has an IDLH of 2 ppm.

All chemical asphyxiants have an impact at concentrations well below what it would take to dilute the oxygen in air to dangerous levels. Even carbon dioxide, which, with an IDLH of 40,000 ppm (4%), most people think of as non-toxic, has an impact at concentrations less than those of simple asphyxiants. A concentration of 8% CO2 can result in loss of consciousness after exposure for 5 to 10 minutes, even though the concentration of oxygen is 19.2%.

Treatment for Asphyxia

The word “asphyxiation” refers to death due to lack oxygen in the body. Since there is no treatment for death, there is no treatment for asphyxiation. However, before death, during asphyxia, there are treatments. They must be timely, though. Only four minutes without oxygen can lead to permanent brain damage. After ten minutes without oxygen, brain death occurs. So, any treatment must restore oxygen to the body and do it quickly.

For engulfment, treatment starts with clearing the airway of any obstruction. That’s what the Heimlich maneuver is for. If clearing the airway is not enough by itself, then it must be followed with cardiopulmonary resuscitation (CPR), or some other means of forcing oxygen into the lungs.

For simple asphyxiants, the airway is already clear of obstructions. The treatment is to supply oxygen. In the simplest cases, moving the victim to fresh air may be enough. Or it may mean CPR in the short term, until oxygen—not air—can be administered through a breathing tube, a mask, or a ventilator.

For chemical asphyxiants, the problem is not that there is not enough oxygen present, but that it is prevented from being transported to where it is needed in the body. This requires medication to either overcome or reverse the effects of the chemical asphyxiants. Typically, the medication is specific to the specific chemical asphyxiant. After administering the medication, it often necessary to follow with oxygen therapy.

Are There Asphyxiants at Your Facility?

You shouldn’t be surprised to discover that your facility has asphyxiation hazards. If they are chemical asphyxiants, you should already be aware of them and have the appropriate treatments on hand. Time is of the essence when responding to asphyxia.

OSHA has waged a vigorous campaign over the years to inform the grain-handling industry of engulfment hazards in silos. However, the kinds of liquids and flowable solids that can result in engulfment are not limited to the grain-handling industry. If your facility is using them, even if they are not toxic, be aware of the engulfment hazard and make sure the policies and procedures to prevent unsafe entry and engulfment are in place and enforced.

The most insidious of all the asphyxiation hazards, however, are the simple asphyxiants. Inerting gases are supposed to make processes safer. So, many lose sight of the fact that they have their own hazards, especially in small, poorly ventilated spaces. Make sure you have measures in place to protect workers from the inadvertent release of simple asphyxiants.

The last thing you want to hear over the radio is an operator gasping, “I can’t breathe.”


  • 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.