Many of the most common toxic gases, including carbon monoxide, hydrogen sulfide, sulfur dioxide, chlorine, ammonia, cyanide, ethylene oxide, nitric oxide, nitrogen dioxide and chlorine dioxide, can be measured by means of compact, substance-specific electrochemical sensors. Gas that enters the sensor undergoes an electrochemical reaction that causes a change in the electrical output of the sensor. The difference in the electrical output is proportional to the amount of gas present.
The sensors are designed to minimize the effects of interfering contaminants on readings. Electrochemical sensors are compact, require little power, exhibit excellent linearity and repeatability, and generally have long life span.
New sensor technologies such as miniaturized photoionization detectors (PIDs) for volatile organic contaminant (VOC) measurement have increased the number of toxic gases that can be measured by means of compact, portable gas detectors. As exposure limits continue to drop, atmospheric monitoring programs increasingly need to include direct quantifiable measurement for many additional toxic substances.
Most obvious hazards: CO and H2S
Carbon monoxide (CO) and hydrogen sulfide (H2S) are still the two most commonly encountered toxic gases. Exposure limits for CO vary widely as a function of jurisdiction and workplace activity. The most widely recognized standards for CO reference an 8-hour TWA of 25 PPM, 35 PPM or 50 PPM, and a Ceiling (peak concentration) of no more than 200 PPM. A concentration of 1,200 PPM is immediately dangerous to life and health (IDLH). A concentration of 1,600 PPM can cause death.
Exposure limits for H2S also vary widely. The most widely recognized standards for H2S reference an 8-hour TWA of 10 PPM or 20 PPM, and a 15-minute STEL of no more than 15 PPM. Concentrations above 100 PPM should be regarded as immediately dangerous to life and health. Exposure limits are likely to be reduced even further in the future. Increasing awareness of the hazards associated with chronic exposure to even low concentrations of hydrogen sulfide has led to a draft proposal by the ACGIH® to lower the workplace exposure limits for H2S to an 8-hour TWA of only 1.0 PPM, and a 15-minute STEL of 5 PPM.
Chlorine and ammonia
Chlorine (Cl2) is a pervasively common industrial chemical. The most widely recognized standards for Cl2 reference an 8-hour TWA limit of 0.5 PPM, and a 15-minute STEL of 1.0 PPM. For many individuals, these concentrations may be too low to detect by smell. The corollary is that if you can smell the odor of chlorine, you are probably at or above the exposure limit for the substance.
Ammonia (NH3) is a highly toxic gas. The most widely recognized exposure limits for ammonia are an 8-hour TWA of 25 PPM, with a 15-minute STEL of 35 PPM. Besides its toxic properties, ammonia is also an explosively flammable gas, with a lower explosion limit (LEL) concentration of 16% volume.
VOCs
Since most volatile organic compounds (VOCs) are combustible at higher concentrations, the tendency in the past was to monitor them by means of the percent LEL combustible sensor included in most multi-sensor instruments. Unfortunately, we know today that many VOCs present a toxic hazard when present at much lower concentrations. For most VOCs, long before you reach a concentration sufficient to register on a combustible gas indicator, you will have easily exceeded the toxic exposure limits for the contaminant.Most combustible gas reading instruments display readings in % LEL increments, with a full range of 0 – 100% LEL. Typically these sensors are used to provide a hazardous condition threshold alarm set to 5% or 10% of the LEL concentration of the gases or vapors being measured. Readings are usually displayed in increments of +1% LEL.
LEL sensors that display readings in +1.0% LEL increments are excellent for gases and vapors that are primarily or only of interest from the standpoint of their flammability. Many combustible gases, such as methane, do not have a permissible exposure limit. For these gases, using a sensor that expresses readings in % LEL increments is an excellent approach.
But many other combustible vapors, such as hexane, fall into a different category. Although VOC vapors may be combustible, and easily measured by means of a hot-bead sensor, they may also have a toxic exposure limit that requires taking action at a much lower concentration.
Using a combustible gas monitor to measure VOCs presents a number of other potential problems as well. To begin with, most combustible sensors have poor sensitivity to the large molecules found in VOCs, fuels and solvents with flashpoint temperatures higher than 100ºF. But even when the span sensitivity of a properly calibrated instrument has been increased sufficiently to make up for this inherent loss of sensitivity, an instrument that provides readings incremented in 1.0% LEL steps cannot resolve changes in concentration smaller than ±1.0% of the LEL concentration of the substance being measured. Because % LEL detectors are poor indicators for the presence of many VOCs, lack of a reading is not necessarily proof of the absence of hazard.
Employing PIDs
When toxic VOCs are potentially present, it is necessary to use additional or different detection techniques that are better suited for direct measurement of VOCs at PPM toxic exposure limit concentrations. Photoionization detectors (PIDs) are increasingly popular for this application. PIDs use high-energy ultraviolet (UV) light from a lamp housed within the detector to remove an electron from neutrally charged VOC molecules. This produces a flow of electrical current proportional to the concentration of contaminant. PIDs are non-specific, that is, they provide a “broad range†indication of all detectable molecules present in the atmosphere being monitored. PIDs are easily able to register readings for most VOCs in 1.0 PPM or 0.1 PPM increments. Some instruments are able to provide readings in parts-per-billion (ppb) increments.Increasing concern with the toxicity of VOCs has led to a number of newly revised TLVs®, including those for diesel vapor, kerosene and gasoline. The ACGIH® TLV® for diesel now specifies an 8-hour TWA for total diesel hydrocarbons (vapor and aerosol) of 100 mg/m3. This is equivalent to approximately 15-PPM diesel vapor. This concentration limit is far too low for detection by means of a standard LEL range combustible sensor.
Diesel vapor has always been regarded as a potential fire hazard but largely ignored as a potential toxic vapor hazard. For diesel vapor, 1.0% LEL is equivalent to 60 PPM. Even if the instrument is properly calibrated for the detection of diesel — which is not possible for many designs — a reading of only 1.0% LEL would exceed the TLV® for diesel by 600 percent!
The best approach
The only way of being sure that toxic contaminants are not present in dangerous concentrations is to look for them with an atmospheric monitor designed for their detection.
The best approach to VOC measurement in many cases is to use a multi-sensor instrument capable of measuring all the atmospheric hazards that may be potentially present. Having a single instrument equipped with multiple sensors means no condition is accidentally overlooked.
