February, 2004
Monitoring for PELs, TLVs of Toxic/Combustible Gases
By Chris Sanford
With neither a definite shape nor volume, and sometimes no color or odor, gases can be hard to detect and sometimes dangerous. Most gases are harmless. Others, such as carbon monoxide, are toxic or flammable and explosive, and very deadly.
A gas consists of individual chemical molecules dispersed in air, such as oxygen at normal temperature and pressure. However, that can all change quickly. When toxic chemicals are present in the workplace, exposure needs to be determined by measuring the concentration of a given chemical in the air and how long it has been there.
A chemical is determined to be a hazard depending on the following factors:
• Toxicity how much of the substance is required to cause harm,
• Route of exposure how the substance enters the body;
• Dose How much enters the body;
• Duration The length of time of the exposure,
• Reaction and interaction Other substances exposed to, and
• Sensitivity How the body reacts to the substance, compared to others.
Inhalation. The most common type of exposure occurs when you breathe a substance into the lungs. The lungs consist of branching airways (called bronchi) with clusters of tiny air sacs (called alveoli) at the ends of the airways. The alveoli absorb oxygen and other chemicals into the bloodstream.
Some chemicals are irritants and cause nose or throat irritation. They may also cause discomfort, coughing or chest pain when they are inhaled and come into contact with the bronchi (chemical bronchitis). Other chemicals may be inhaled without causing such warning symptoms, but they still can be dangerous.
One such chemical is carbon monoxide, CO, which will always be the number one gas of interest to the public, says Michael D. Shaw, executive VP and director of marketing for Interscan Corp., a maker of toxic-gas detection equipment.
More people are affected by carbon monoxide than any other compound, said Shaw, though not necessarily in manufacturing, where hydrogen sulfide has become a compound of great concern in industrial settings.
Two types of gas detection
There are two types of gas detection in industry, toxic gas detection and combustible gas detection. Gases that are combustible must be monitored for explosiveness. This has been going on for a long-time, said Shaw, but it is generally percent levels that were observed. With toxic gases, the permissible exposure levels were ratcheted down quite a bit, with measurements in parts per million, as opposed to percentages.
With a change in need, companies were making their own sensors, until in the 1990s when the market became commoditized,’ said Shaw, and different manufacturers were selling electrochemical toxic gas sensors made by the same company. The proprietary nature of the industry changed, and a series of interchangeable portable gas detectors using primarily the same sensors flooded the market.
Though most of these manufactured sensors were very good for the average application, Shaw says there became an overemphasis on portables, adding that there is more to life than portable instruments when it comes to gas detection. They should act as a bridge between survey work and discovering what has to be installed for continuous monitoring, he said.
Shaw cited one manufacturer’s informal motto, First detection, then protection.’ However, he added that over a period of testing, it is not unusual to find areas that will always have some gas. Then it is time to put in a continuous system with 24/7 monitors, data recorders and alarms.
Whatever the setting, though, low oxygen will kill you in seconds, and carbon monoxide is not the only compound of concern to industrial hygienists.
Modern gas detection started as far back as the1920s and was driven by the mining and steel industries. Coal miner’s canaries were the first gas detection tools, but who really knew if the canary died of old age or a toxic substance in the air.
Commonly Used Terms
• PPM: Parts per million
• LEL (Lower Explosive Limit): the lowest concentration of a gas in air, which, when ignited, will sustain combustion.
• TLV (Threshold Limit Value): Refers to airborne concentrations of substances, and represent conditions under which nearly all workers may be repeatedly exposed day after day without adverse health effects.
There are three categories of TLVs:
• TWA: Time Weighted Average
This refers to a time weighted average concentration for a normal eight-hour day, in a 40-hour workweek in which most workers can be exposed repeatedly without adverse effect.
• STEL: Short Term Exposure Limit
If the TWA has not been exceeded then the STEL is the concentration in which most workers can be exposed continuously for a short period without suffering from irritation, chronic or irreversible tissue damage or narcosis to the degree that would impair self-rescue, work efficiency or cause accidents. Further, a STEL is a 15-minute TWA that should not be exceeded any time during a normal workday even if the worker is within the 8-hour TWA. Exposures above the TWA and up to the STEL should not be longer than 15 minutes and should not exceed four times a day.
• Ceiling: This concentration should not be exceeded during any part of the working exposure.
Applications Engineering
Industry today requires engineering, said Shaw, adding that gas detection is not an easy thing to do. Industrial hygienists need to worry about more than just the popular compounds circling about. With portable instruments, there is no application engineering, he said, and you wind up with a scripted approach to handling what could be a very individual problem.
“Most problems are unique,” added Shaw. A commoditized product may not always give you what you need.”
As part of an engineered application approach, the first thing to do is to determine the gas or gases of interest, and the desired full-scale measuring range of each.
List any potentially interfering gases that may be present. This will be useful information for the applications people; decide if the application is for portable survey monitoring, or for a fixed, continuously operating monitoring system.
Then determine if you would like some form of data acquisition or recording.
All gas detection instruments must be calibrated against a known gas standard, and will have other maintenance requirements. Proper calibration is about 90 percent of successful gas detection.
Application Types
Workplace Monitoring: Refers to measuring the INDOOR work environment, for monitoring worker safety, for OSHA compliance or industrial hygiene surveys. The appropriate instrumentation to be recommended in this case are monitors designed with the intent to be sensitive and accurate to address TLV levels and below. Generally, these monitors are NOT designed to measure high concentration levels. It may be of interest to monitor combustible gases in the workplace, to make sure that their concentrations are well below the LEL.
Process Monitoring: Refers to an application where a constant stream of gas will be present. In most of these cases, the customer will want to measure a change in the gas concentration, for process control, rather than personnel safety reasons.
Stack Monitoring: Applications refer to monitoring stack emissions, to meet EPA compliance.
Further Defining the Application
• Do you need a portable, direct-reading survey monitor for occasional spot check monitoring, or rather a stationary, installed area monitor to operate on a round-the-clock basis?
• Would you like data acquisition or strip chart recorder capability? This can be especially important to fend off lawsuits—not to mention ensuring the safety of your workers and the people who use your facilities.