Coping with the New TLV for Diesel Fuel

Long before you reach a concentration of diesel vapor sufficient to register on a combustible gas indicator, you will have easily exceeded the new TLV.

• By Bob Henderson, Edward J. Willwerth
• Feb 01, 2003

DIESEL fuel is among the world's most important commodities. The United States literally runs on diesel and the other closely related "middle distillate" fuels, such as home heating oil, coal oil, gas oil, and marine diesel fuel. During August 2002, daily consumption of diesel fuel and related products in the United States averaged more than 150,000,000 gallons per day.1

Over the last several years, most of the attention paid to the presence of diesel fuel in the workplace has centered on diesel exhaust. Diesel exhaust has been increasingly suspected as being carcinogenic and a pervasive source of particulate pollution associated with lung disease. Diesel fuel is also a moderate fire hazard, and many safety programs have long included using portable gas detectors to measure its vapor concentrations present in confined spaces or other workplace atmospheric environments. With the advent of a new Threshold Limit Value (TLV®) by the American Conference of Governmental Industrial Hygienists (ACGIH), however, diesel fuel must now be considered to have some important new toxicity considerations.

The 2002 Edition of the ACGIH Threshold Limit Values for Chemical Substances and Physical Agents includes a new exposure limit for diesel fuel. This TLV® specifies an eight-hour time weighted average (TWA) for total diesel hydrocarbons (vapor and aerosol) of 100 mg/m3. This is equivalent to approximately 15 parts-per-million diesel vapor.

Diesel vapor has always been regarded as a potential fire hazard but largely ignored as a potential toxic vapor hazard. The new TLV® has the potential for completely changing the techniques used to monitor for the presence of diesel vapor. With a typical lower explosive limit of 0.6 percent (6,000 ppm), long before you reach a concentration of diesel vapor sufficient to register on a combustible gas indicator (usually 1 percent LEL, or about 60 ppm), you will have easily exceeded the new TLV.

Diesel's TLV has not been introduced without controversy. ACGIH's diesel fuel committee's documentation states, "Of note, no unique aerosol inhalation hazard to human health has been demonstrated over many decades of diesel use as fuels." Nevertheless, in establishing the TLV's level, it states, "The diesel fuel TLV is derived from the rodent inhalation studies and the application of interspecies uncertainty factors: therefore, a TLV-TWA of 100 mg/m3 of total hydrocarbons as aerosol/vapor is recommended." ACGIH therefore seems to have chosen to ignore over a century's epidemiological data and opted for animal studies. The new TLV is far more conservative than the U.S. Navy's relatively recent (1996) and well-documented exposure guidance "Permissible Exposure Levels for Selected Military Fuel Vapors." In it, the Navy recommends a TWA of 350 mg/m3, or approximately 50 ppm.

Why Do TLVs Matter?
Occupational Safety and Health Administration regulations use the term Permissible Exposure Limit to define the maximum concentration of a listed contaminant to which an unprotected worker may be exposed during the course of his workplace duties. PELs usually are expressed as an eight-hour, TWA exposure limit. Exposure limits for gases and vapors are usually given in units of parts per million or mg/m3. Limits for mists, fume, and particulate solids are expressed in units of mg/m3.

The TWA concept is based on a simple average of worker exposure during an eight-hour day. The TWA concept permits excursions above the TWA limit only as long as they do not exceed the short term exposure limit or ceiling (if established) and are compensated by equivalent excursions below the limit. The regulatory TWA is a calculation using sampling results projected to a full eight hours. Time not measured is projected as zero exposure in many jurisdictions. Thus, if a worker is exposed to 100 ppm of contaminant for four hours, the eight-hour TWA would be equal to only 50 ppm at that time.

OSHA PELs are listed in Subpart Z (Section 1910.1000) of the Code of Federal Regulations. Subpart Z contains three tables (Z-1, Z-2, and Z-3) that list the contaminants specifically regulated by this standard. The federal PELs set the highest allowable unprotected workplace exposure limits for these substances. Individual states follow either the federal regulations or their own, state-specific permissible exposure limits. States may not publish or follow exposure limits that are more permissive than federal OSHA's limits.

The ACGIH TLVs are among the world's most widely used and respected guidelines for workplace exposure to toxic substances. TLVs are developed and designed to function as recommendations for the control of health hazards and to provide guidance intended for use in the practice of industrial hygiene. Although ACGIH TLVs are not expressly developed for use as legal standards, they are frequently incorporated by reference into state, federal, and many international regulations governing workplace exposure. They may also be cited or incorporated by reference in consensus standards of organizations such as the National Fire Protection Association (NFPA) and American National Standards Institute (ANSI).

Given the potential for lawsuits, many employers have made the strategic decision to base their corporate health and safety programs on conservative applicable recognized standards. Because ACGIH recommendations are frequently more conservative than OSHA PELs, many programs, especially the programs of multi-national or prominent corporations, use the ACGIH TLVs.

What is Diesel Fuel?
Diesel fuel is not a pure compound or single chemical substance. Diesel fuel is a mixture of hydrocarbons that have certain flammability characteristics and chemical properties. In fact, the specific individual chemical components in diesel fuel can vary widely from batch to batch. What is (or should be) highly consistent from batch to batch are the performance characteristics used to define what is sold as diesel.

Diesel fuels are produced from distillates of crude oil that fall between the boiling temperatures of gasoline (175º C) and lubricant oils (338º C), thus their name as "middle distillates." Middle distillate fuels are mixtures of hydrocarbons (saturated alkanes, olefins, and aromatics including benzene) with molecules ranging from nine carbons to 20-plus carbon atoms in size.

Diesel No. 2 usually contains a small but measurable concentration of benzene (normally on the order of 0.02 percent). Marine diesel is a special grade of fuel oil that is more viscous than No. 2 diesel, has a higher sulfur content (up to 2.0 percent), and is associated with higher concentrations of aromatics. Jet fuel, kerosene, diesel fuel, and heating oil are included as middle distillates, but the new TLV does not apply to these fuels.

Basically, anything that burns within its specifications and runs a diesel engine can be and is sold as diesel. The following table lists the relative proportions of categories of hydrocarbon molecules found in diesel fuel produced by two different refining methods. Although the components and ratio of components vary widely, both fuels are sold as No. 2 diesel fuel.

Chemical and physical properties of diesel fuels include:

Diesel Fuel Toxicity
The toxicity of diesel fuel depends on its physical form, dose, and route of exposure. Swallowed or ingested diesel fuel can be quickly fatal even with relatively small doses. Prolonged skin contact can lead to dermal irritation and is associated with increased risk of skin cancer in some laboratory animal studies.

Diesel can be inhaled in the form of fine aerosolized particles or droplets, or as vapor given off by the diesel fuel as it evaporates. According to the toxicological studies cited by the ACGIH, diesel hydrocarbons are predominantly absorbed from the vapor state in humans.

Diesel fuel vapor is relatively benign in terms of its acute toxicity. In other words, few workers suffer immediate harm as a function of exposure to high concentrations of diesel vapor. On the other hand, diesel fuel has shown consistent evidence of carcinogenicity and tumor generation in mice and other animal studies. The U.S. Navy recognizes several potential hazards of middle distillates:

• Dermatitis
• Mist inhalation, chemical pneumonia, pulmonary edema
• Demonstrated low potency animal carcinogen
• CNS depression at high vapor level
• Some reported hepatoxic action at high levels

Diesel fuel has a relatively low vapor pressure, which means that it needs to be warm in order to produce high or potentially combustible concentrations of vapor. Even cold diesel, however, is more than capable of producing vapor concentrations that exceed the new 15 ppm ACGIH TLV. With a typical vapor pressure of 2.3 mmHg at 21°C, one would expect a concentration of approximately 3,000 ppm vapor in a saturated atmosphere--200 times the new TLV. Whatever the merits of the new TLV regarding vapor, its implications are significant.

Techniques Used to Measure Diesel
Three techniques are commonly used to measure diesel vapor: colorimetric detector tubes, combustible gas monitors that use catalytic "Hot Bead" combustible gas sensors to detect vapors in percent LEL or ppm ranges, and photoionization detectors (PIDs).

Colorimetric detector tubes
Colorimetric measurement techniques utilize reagents that undergo a color change when exposed to the specific contaminants they are designed to detect. Colorimetric detection is a real-time measurement technique. It can be used both for broad-range screening, as well as for analysis of specific contaminants. Several manufacturers offer a low range total hydrocarbon detector tube that can be used for the measurement of diesel vapor and is capable of obtaining readings at the new ACGIH TLV.

Colorimetric detector tubes resemble short glass straws packed with fine grain silica gel, activated alumina, or some other medium. The tube contents are impregnated with reagents that undergo a color change reaction when exposed to specific contaminants. To use, the ends are broken off and the tube is inserted into a pump designed to pull a calibrated volume of air through the tube. Contaminants in the air that can react with the reagent produce a color change progressively along the tube as the air passes through. The length of the color change is proportional to the amount of contaminant present.

The outside of the tube usually includes a measurement scale for estimating concentration, as well as other information such as the sample volume and number of pump strokes required to obtain a reading, the direction of flow when the tube is properly installed in the pump, and measurement units in which the reading is expressed. The reading is taken as the furthest distance along the tube that the color change just becomes visible.

Detector tubes do not provide dynamic, real-time measurement when concentrations and conditions are subject to change over time. Each time a reading is desired, it is necessary to insert a new tube into the pump and take a new "snapshot" of the current concentration. Also, diesel vapors, being substantially heavier than air, tend to collect in low spots or become localized in certain areas of the environment being monitored. It is easy to miss a concentration "hot spot" when using tubes to obtain readings.

Another limitation is the ± 25 percent of length of stain that is normally the stated accuracy of detector tubes used to measure diesel and other hydrocarbon vapors.

Nevertheless, detector tubes for total hydrocarbon are available and may provide a viable approach for measurement of diesel vapor at the new TLV. Most hygienists and safety managers already possess detector tubes and pumps for other purposes, so buying the necessary additional low range total hydrocarbon tubes necessary to measure diesel is an easy and inexpensive approach.

Combustible gas monitors using catalytic bead sensors
Instruments for monitoring ignitable mixtures usually use catalytic (hot bead) sensors to measure the gas. The hot bead sensor contains two beads wired into opposing arms of a balanced Wheatstone Bridge electrical circuit. The "active" bead is treated with a platinum or palladium-based material that allows catalyzed combustion to occur on the bead. The "reference" (or compensator) bead in the circuit lacks the catalytic outer coating but in other respects exactly resembles the active bead. If ignitable gas or vapor is present, oxidation will heat the active bead to a higher temperature than the reference bead. The temperature of the untreated reference bead is unaffected by the presence of gas. The difference in temperature between the beads is proportional to the concentration of combustible gas.

Using a combustible gas monitor to measure diesel fuel presents a number of problems. To begin with, most combustible sensors have poor sensitivity to the large molecules (C9 and higher) found in diesel fuel mixtures. 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 reads in percent LEL, with readings incremented in 1.0 percent LEL steps, cannot resolve changes in concentration smaller than ± 1.0 percent of the LEL concentration for the substance being measured.

The LEL concentration of diesel fuel ranges from 0.6 percent to 1.3 percent volume in air. This is equivalent to 6,000 to 13,000 ppm. This means it takes a minimum concentration of between 60 and 130 ppm of diesel fuel vapor to produce even a 1.0 percent LEL reading! It is simply not possible to use a percent LEL combustible gas monitor to verify compliance with the new diesel TLV. Percent LEL combustible gas monitors are just not sufficiently sensitive.

Because percent LEL detectors are usually poor indicators of the presence of diesel vapor, lack of a reading is not necessarily proof of the absence of hazard. An instrument that is better at direct measurement of diesel vapor, and which is capable of resolving vapors into the ppm range, is the more appropriate approach.

Another very important consideration is that combustible gases and vapors other than diesel may also be present simultaneously. Although catalytic bead sensors may have limitations with concern to the measurement of diesel vapor, they are by far the most widely used and dependable method for measuring methane and other combustible gases and vapors with smaller, lighter molecules.

Photoionization detectors
Photoionization detectors use high-energy ultraviolet light from a lamp housed within the detector as a source of energy used to remove an electron from the neutrally charged target molecules. The electrically charged fragments are called ions. PIDs collect the charged particles on charged plates. This produces a flow of electrical current proportional to the concentration of contaminant.

The amount of energy needed to remove an electron from the target molecule is called the ionization potential for that substance. The larger the molecule, or the more double or triple bonds the molecule contains, the lower the IP. Thus, in general, the larger the molecule, the easier it is to detect! This is exactly the opposite of the performance characteristics of the catalytic hot bead-type combustible sensor.

Photoionization detectors are easily able to provide readings at or below the new 15 ppm TLV for diesel. Most PIDs are capable of providing 0.3 ppm resolution for diesel.

Unlike detector tubes, PIDs provide real-time, dynamic readings that are able to capture sudden changes in the concentration of diesel and other VOC vapors in the area being monitored, and which can easily be used to track down "hot spots" or localizations in the concentration of vapor.

Multi-sensor detectors
Catalytic hot bead combustible sensors and photoionization detectors represent complementary, not competing detection techniques. Catalytic hot bead sensors are excellent for the measurement of methane, propane, and other common combustible gases that are not detectable by means of a PID. On the other hand, PIDs can detect large VOC and hydrocarbon molecules that are effectively undetectable by hot bead sensors, even when they are operable in ppm measurement ranges.

Focusing too tightly on diesel may increase the likelihood of missing other hazardous conditions. The best approach to diesel 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.

In the past, photoionization detectors have tended to be bulky, temperamental and expensive. This has changed dramatically over the last few years. Today, compact, multi-sensor designs that include LEL, O2, and electrochemical toxic sensors, as well as a miniaturized photoionization detector, have allowed this very useful detection technique to be included in many confined space, hazmat, and environmental monitoring programs.

Conclusion
The new ACGIH TLV® for diesel is now in effect. In many cases, the new TLV was incorporated into exposure limit compliance regulations immediately upon publication. In other cases, there is a grace period still in effect.

In all cases, prudent safety managers should evaluate their existing programs for diesel vapor measurement and make sure the procedures in place comply fully with all applicable standards. Most important, safety managers need to ensure affected workers are adequately protected in light of these new recommendations. It almost certainly means changes in the way you are currently measuring diesel fuel vapors.

References
1. Oil & Gas Journal
2. 2002 ACGIH Diesel Fuel TLV®

This article originally appeared in the February 2003 issue of Occupational Health & Safety.