The OSHA standards list permissible exposure limits (PELs) for about 600 chemicals, such as acetone, methyl ethyl ketone, toluene and ethyl alcohol, that are commonly found in the industrial environment. Although OSHA has airborne limits for these substances, the agency does not specifically require that air sampling be performed to evaluate employee exposures to most of these substances. Instead, it simply requires that employee exposures to the regulated chemicals remain below the PEL. Although the most practical way of making this determination is to perform air sampling, sampling is not mandated for the vast majority of the substances that OSHA regulates.
However, there are a handful of substances (Table I) that specifically require that air sampling be performed. Moreover, these substance-specific standards require that periodic sampling be performed on a regular basis such as monthly or quarterly. Some of the substances such as lead, cadmium and methylene chloride may be found in many workplace settings.
Because the samples are collected at the employee’s nose and mouth, they are called “breathing zone” samples. The breathing zone can be visualized as a hemisphere about 6 to 9 inches around the employee’s face. Breathing zone samples provide the best indication of the concentration of contaminants in the air the employee is actually breathing. Two types of instruments are commonly used to do personal breathing zone sampling: passive monitors and personal sampling pumps.
Passive Monitors Passive monitors are small plastic enclosures about half the size of a pager. They are filled with a granular solid sorbent such as activated charcoal that has an affinity for organic gases and vapors. One section of the enclosure is open to the air. Organic gases and vapors in the air that pass through the opening by diffusion are adsorbed, or trapped, by the sorbent material.
At http://www.dol.gov/elaws/osha/lead/glossary.asp OSHA defines the breathing zone as the area “within a 10-inch radius of the worker’s nose and mouth.” That would indicate that an instrument used primarily for personal protection from toxic hazards such as H2S should be worn on the collar, the lapel, on a breast pocket or even on the brim of a hard hat – or simply within a 10-inch radius of your nose and mouth.
All exposures are considered without regard to respiratory protection. In other words, if the employees being sampled are wearing respirators, the protection afforded by the respirator is not taken into account when considering the level of exposure.
Accuracy of the measurements. Substance-specific standards typically stipulate the level of accuracy that the sampling and analytical method must meet.
Initial monitoring. Initial or baseline sampling must be conducted to determine the existing level of exposure. The results of this monitoring are used to establish the frequency of periodic monitoring and may invoke other requirements of the standard, such as medical surveillance, protective equipment and written compliance plans.
Periodic monitoring. The frequency of periodic monitoring varies from substance to substance and is based on where the measured exposure is relative to the action level or PEL. The vinyl chloride standard, for example, requires monthly sampling for any employee exposed above the PEL. The benzene standard, on the other hand, requires annual sampling for employees exposed above the action level but below the PEL, and sampling every six months for any employee exposed above the PEL.
Termination of monitoring. Results of the periodic monitoring are used to establish when monitoring may be discontinued. For example, the lead and cadmium standards permit monitoring to be discontinued when two consecutive samples taken at least seven days apart are below the action level.
Additional monitoring. The standards include a provision for conducting additional monitoring whenever there has been a production process, control or personnel change, or when there is reason to suspect other change which may result in a new or additional exposure.
Some would suggest that because gases like H2S are heavier than air that the instrument used to protect against them should be worn lower on the body, around the knees or attached to the top of the boot. While there may be some validity to this argument, I believe that this puts the instrument itself in danger of being damaged in the working environment or even lost without notice and may make it more difficult to recognize that the instrument is alarming in high noise areas.
Don’t ignore the fact that the GasBadge Plus or any single toxic gas monitor is intended to provide direct protection from a respiratory hazard. Keep breathing, keep safe, and keep it within the “breathing zone”.
But my monitor sometimes show NEGATIVE NUMBERS
All electrochemical or catalytic gas sensors can be prone to both positive and negative drift due to environmental factors such as changes in temperature and humidity. However these are not the most common causes of negative sensor readings.
Negative sensor readings more commonly occur when your instrument has been “zeroed” in a contaminated atmosphere, where small levels of the sensors’ target gasses are present. When the instrument is later in a clean-air environment, the sensors will show a negative reading that corresponds to the concentration on the contaminant that was present during the zeroing operation. For example, if there is 5 PPM carbon monoxide present when the sensor is zeroed, the reading will be -5 PPM when the sensor is returned to clean air.
Negative readings may also occur when the sensor is exposed to a gas that produces a negative cross interference. If a sulfur dioxide sensor, which typically has a -100% cross interference to nitrogen dioxide is exposed to 2 PPM NO2, the resultant sulfur dioxide reading on your instrument will be -2 PPM.
So, does this mean that you should avoid using sensors that have negative cross interferences to each other in the same instrument? Absolutely not! If you have NO2 and SO2 present in the same atmosphere, the only way that you can understand the true concentration of each gas is by having both sensors. In the example that we used above, if your atmosphere contained 2 PPM SO2 along with the 2 PPM NO2, the resultant SO2 reading due to the negative cross interference would be zero. The only way that you could know that you have 2 PPM SO2 present is by recognizing the presence of the NO2 gas and understanding its effect on the SO2 sensor. Eliminating one of the sensors from the instrument does not eliminate the hazard that it, and you, are being exposed to!
Customers will sometimes say that they have never seen a negative reading on an instrument before but they have recently changed monitors and now seem to see them all the time. This is because some manufacturers believe that negative readings only confuse users and mask them from their instruments. All negative readings are displayed as zero. This is a practice that can in fact serve to mask you from seeing and recognizing the hazards that exist. If an H2S sensor has an offset of -10 PPM due to drift or a false zero operation that has been masked by the manufacturer, exposure in a true concentration of +10 PPM will still produce a zero reading and a concentration of +20 PPM would only be displayed as 10. This situation would be easier to recognize if the negative reading was normally displayed in the first place.
So, while negative readings are puzzling and uncomfortable to most gas monitor users, they are not always a bad thing. If you understand the circumstances that cause the negative readings, you will get more information from your instrument and have a better understanding of the environment you are working in.