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It is a common principal in response that where you are standing is critical to success when handling hazardous materials. Recognizing the degree of danger of hazardous materials will change the way an emergency responder functions. No one sits on a box of burning explosives, or stands in the center of a busy interstate highway because we know it's dangerous. Many of us will enter a tank or approach a leaking cylinder without regard to our own safety. In order to respond adequately we must weigh the risk to our own personal exposure against the benefit of the response effort. This balance is to say the least, delicate, and is in no way easy. Recognizing the degree of hazard not only by it characteristics and hazards but quantified by the amount or concentration can make a world of difference to the ultimate outcome. It is a simple premise - Where you are standing in a chemical emergency dictates the outcome. It is a three position emergency perception. Ask yourself - Is it?:

    • SAFE
    • UNSAFE
Safe for our purposes means no harmful effects from chemicals exist at the present time. Unsafe atmospheres are areas or concentrations that if you stay for a prolonged period of time something will happen. Dangerous areas are situations that are an immediate threat to life and health. This atmosphere could be deadly and result in catastrophic events.


What is safe? Many people would say, No chemicals therefore - safe. Emergency response and even society can not have that simplicity. Chemicals are everywhere and are usually safe if they are being used as they were intended. They are in a Controlled state and are not being used carelessly. Emissions from them in a work place are monitored under controlled can be considered safe. These areas have been classified safe because we are below the PEL permissible exposure limits by OSHA. Other safety agencies such as NIOSH’s REL’s Recommended Exposure Levels and ACGIH call them TLV’s or Threshold limit values. Generally these are amounts or values that are the amount of exposure or an average work day (usually 8) for a work week (usually 40). Some are calculated on basis of ceilings or not to exceed level. Simply put it is Safe to Breath.

Whether you use Ceilings, Time Weighted Averages or Short Term Exposure Limits, All these exposure limits have one thing in common, remain below them and the exposure is considered relatively safe. Of course that is Safe by all known information today. Exceed the recommended levels of any of these three and now you are no longer safe! Therefore you must be unsafe. Personal Protection must be used. Since the common responder does not carry instrumentation that can measure these levels it is better to use visible clues of safe. For example, are the chemicals in the container? Are they in the Controlled (SAFE) or Uncontrolled, spilt, released (UNSAFE) condition for determining an initial beginning point to responding. In the beginning of every hazardous materials emergency assume at least it is unsafe and try to eliminate atmospheres that are considered dangerous. Never let your guard down, if it's spilt consider it unsafe. When in doubt it’s unsafe!


If it's not in a controlled state, It is not normal, and therefore it's by default unsafe. Anytime a chemical has gotten out of its container it will probably exceed any of the Worker Safety Measurements and there must be an increased level of hazard. Obviously there must be some form of protection needed. Most of the time it is usually respiratory protection but some skin protection may be needed if the substance is a contact hazard.

The key hazard at unsafe level, the injuries are not of the fatal type because the concentration or length of exposure is usually short. Most are reversible, Head Aches, nausea, irritation to eyes, nose, or throat. The real hazard of unsafe is that the longer or more you are exposed to any substance the greater harm. Reversibility is also linked to the length of exposure. Any emergency response will be unsafe when the concentration rises or the length of exposure. It will go from unsafe to dangerous if the exposure (Time) or Concentration (Amount) grows excessively. This relationship of concentration and time as it is related to harm or dose is the heart of the dose/response curve. Harm is directly related to the concentration times the length of exposure or as Harm equals Concentration * Time (H = C x T).

Alcohol is a good example of this relationship. Drink more or faster and greater harm occurs. Alcohol Effects are reversible in the early stages but in large enough concentrations or after long term exposures (chronic) it can be lethal.


The biggest question in emergency response is why does one chemical have the potential to kill or is lethal when sometimes the same chemical is spilled and responders clean it up with little or no problems, sometimes, although erroneously without respiratory protection. Because the concentration is not reached the dangerous level. But dangerous changes with each chemical. Some are dangerous because of fire, explosion, poison or corrosive. One commonality of all chemical exposure, the more you get the worse it gets.

When concentration increase above these unsafe levels life threatening injuries can occur.

Immediate monitoring atmospheres or visual clues are vital to life safety when the situation is dangerous. It is important to know when the concentrations are dangerous or life threatening. In addition the time limit is drastically reduced. First let’s ask, What is dangerous? Any definition of dangerous includes situations that are:

    • Death
    • Irreversible Injury
    • Interferes with Your ability to Escape -

Impaired judgment

Using this definition dangerous is BAD (Big Awful Disasters). Remember keep it simple When chemicals are released only two things can come out. Energy or Matter. Energy is almost always dangerous. As responders we have always disliked any energy situation. Bulging containers, burning dynamite, over pressuring tanks are always considered bad. Energy is any emergency is always dangerous, simply because all the previous issues are present if energy occurs, irreversible injury, death or escape impairing events. Matter is dangerous in concentration or Time.







Thermal Extremes





Exposure to matter at a dangerous level has several definitions all are called Immediately Dangerous to Life and Health (IDLH) but all possess the dangerous characteristics of dangerous listed above.

Immediately Dangerous To Life and Health - (NIOSH)

"The maximum level from which a worker could escape without escape impairing symptoms or irreversible health effects within 30 minutes."

Immediately Dangerous to Life and Health - (OSHA)

" The airborne concentration of any toxic, corrosive or apshyxiant substance that poses and immediate threat to life, or causes irreversible or delayed effects or interferes with a responder’s ability to Escape the atmosphere"

Immediately Dangerous to Life and Health - Occupational Safety and Health Guidance Manual for Hazardous waste Site Activities NIOSH/OSHA/USCG/EPA

"Regardless of the definition... All IDLH values indicate those atmospheres from which escape is possible without irreversible harm should respiratory protection fail."

If it isn’t an energy situation there are three general IDLH Atmospheres most responders are exposed to:

    1. Toxic
    2. Flammable
    3. Oxygen Deflections (Enriched and Decreased)

IDLHs are conservative in the side of safety. Understanding the consensus definition makes the three IDLHs now more clear.

IDLH - Toxic

Chemicals you wouldn’t even think of as lethal poison Ethanol (liquor) has an IDLH, however it is very high because we tolerate it so well. Extremely toxic substances have IDLH’s very low. Any time you are or suspect you are above the IDLH it is dangerous-irreversible at the least and fatal at the worst. These IDLH’s are Toxic. Treat them as such Examine the following poison lines these are graphic illustrations that s toxic concentrations rise and the TLV’s are exceeded harm occurs but they are reversible above the IDLH dangerous exposures occur, some levels above the IDLH are lethal.


IDLH - Flammable

Many chemicals that are flammable have two IDLH’S Toxic and Flammable. It is a fact that when a chemical has reached it’s LEL it has been well above its (toxic) IDLH. The IDLH for toxic can be eliminated with a positive pressure Breathing Apparatus. But in a high concentration of Carbon Monoxide or Methanol responders could be burned to death before dying of the toxicity. The simplest method for determining flammable dangerous is to accept the premise we do not exceed 10% of the LEL. Responders are critically aware of the discrepancy in calibration and relative responses. 10% of LEL on the combustible gas indicator. Is dangerous. Never use the LEL as dangerous, its explosive (What does the "E" in LEL stand for), Explosion is energy and energy is always dangerous. Examine the poison lines below the most important issue is although the concentration has not reached the LEL or even 10% of the LEL it is well above the toxic exposure limits. Simple premise is any chemical that is toxic and flammable (all the organic solvents are) toxic hazards always precede flammable hazards.


IDLH - Oxygen

As any responder knows we need 21% oxygen to carry on normal activities. Below 19.5% the first affected organ is the Brain. You begin to have impaired judgment. This would be critical to your recognizing you’re in danger. Not knowing that you have to leave is a sure sign that further injury will occur. Many rescuers can be overcome in a few breaths breathes of anoxic (low oxygen) atmosphere. Low oxygen atmospheres are the most common IDLH encountered and the most responsible for deaths of responders. The Oxygen line shows the increasing deviation of an oxygen deficient atmosphere.

Many focus on low oxygen atmospheres but oxygen enriched atmospheres are just as dangerous. These atmospheres although not common seriously increase the fire hazard. Oxygen enriched atmospheres are dangerous because they possess fire and explosion hazards. A safety tip is oxygen atmospheres do not exist under normal conditions. Responders encountering any oxygen increased situation should use extreme caution.


Any of these atmospheres are recognizable if instrumentation is available. A specific reading will give responders immediate feedback. But too often there is no ready numbers early in an incident. We then must use our common sense and look for visible indications of dangerous. First assume there are only two locations as pill can occur. Inside and Outside. This at first seems simple, but upon closer examination it is the key to IDLH. To be IDLH outside requires large quantities of matter or possess energy. The only exception is extreme poisons can be dangerous.


Visible Vapor Cloud
Large Container or Vessel involved in the Emergency
Large liquid leaks
Below grade victim or release (Inside or Outside)


Confined spaces (build up of small amounts of vapor to high levels)
Below grade victim or release (Inside or Outside)
Artificial or Natural Barriers (Vapors Trapped)

Common Sense

Biological Indicators, victims, dead foliage, animals
Highly Toxic Chemicals (Very Low IDLH’s Very Low PEL’s)
Heavy Odors or Strong Sensory Warnings
Energy Is Always Dangerous

Hierarchy of Dangerous

Although all dangerous situations possess deadly consequences, there is a field hierarchy that we should respect.

    Flammable or Oxygen Enriched (Both Explosive)
    Low Oxygen


Always be on the look out for any of these situations. An IDLH situation will be cause for difficult decisions to be made. There is a rough degree of danger hen dealing with IDLH situations. This is not etched in stone but serves as a guide. Low oxygen is common but it is the easiest to protect from simple SCBA eliminates the hazard. Unknowns usually turn out to be benign but they cause great concern until identified. Energy usually is a higher degree of danger.

Although Rapid intervention is sometimes necessary by responders if the victim can be saved. However, the numbers of victims must not increase after the arrival of the first responder on the scene. Your involvement must favorably change or alter the outcome for the better. You must be part of the solution not increase the problem.  

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All information © Mike Callan, 2001, unless otherwise indicated