ESLI technology

April 30, 2003
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Working recently with an industrial hygienist at a facility where we were doing some respiratory training, hydrogen sulfide (H2S ) was prevalent in the atmosphere. Despite this, workers we interviewed told us that they could not smell the H2S, which was at concentrations that nearly made us nauseous.

Hydrogen sulfide has an offensive "rotten egg" odor that is easily detectable by smell even at low concentrations. However, with prolonged exposure one can become accustomed, or desensitized, to its smell. This occurrence is known as olfactory fatigue. Workers that become desensitized may not be aware of their exposures above permissible levels, which could lead to respiratory health concerns.

Changes in the air

Because "sensing" contaminants is subjective, the Occupational Safety and Health Administration (OSHA) made changes in the regulations for air-purifying respirators (APRs) several years ago. Prior to the change, users of air-purifying respirators discarded their cartridges when they sensed breakthrough of gases or vapors. The change in regulations eliminated this subjective sensing-of-contaminants method for APR cartridges in non-IDLH (immediately dangerous to life and health) environments. The new regulations required an objective method of determining cartridge service life.

Either of two objective methods specified could be used:

1) The respirator is equipped with an end-of-service-life indicator (ESLI) certified by the National Institute of Occupational Safety and Health (NIOSH), or

2) Alternatively, if there is no ESLI appropriate for the conditions in the workplace, a cartridge change schedule based on objective information could be implemented to ensure cartridges would be changed before the end of their service life.

At the time of this change, the technology for end-of-service-life indicators was not available for direct and accurate determination of cartridge change times, forcing companies to rely on objective information to stay in compliance. Relying on objective information is not an exact science, as variables such as the contaminants, their concentrations, temperature, humidity and the workers' breathing rates must all be considered. In addition, creating and administering cartridge change schedules can be tedious and time-consuming.

Cartridges now equipped

Today, a second, more easily implemented objective method is available for determining the end-of-service-life for APR cartridges. Cartridges equipped with end-of-service-life indicators (ESLIs) are available to protect respirator users against certain acids, ammonia, and toluene diisocyanate (TDI). Similar ESLIs for other substances are under development.

When an air-purifying respirator is recommended for any of the following contaminants: Ammonia, Hydrogen Chloride, Hydrogen Fluoride, Hydrogen Sulfide, Sulfur Dioxide or Toluene-2,4-Diisocyanate (TDI) - an ESLI may be used. (An APR would be recommended at levels below the IDLH value and at or above the OSHA permissible exposure limit (PEL) or the threshold limit value (TLV) published by the American Conference of Governmental Industrial Hygienists (ACGIH).) At or above the IDLH value, a self-contained breathing apparatus or pressure demand airline respirator with an emergency escape cylinder must be used.

How ESLIs work

ESLIs that recently became available have a colorimetric strip positioned inside the clear "see-through" cartridges. The colorimetric strip accurately reflects the degree of contamination and the remaining effectiveness of the adsorbent carbon media inside the cartridge, gradually turning color when exposed to certain contaminants. In the case of ammonia, for example, the color changes from bright yellow to blue. A picture on the outside label indicates to the user that when the blue color on the inside indicator reaches the level of the blue border on the label outside, the service life of the cartridge is about to expire. Both respirator cartridges then should be replaced.

This technology eliminates the need for developing and maintaining cartridge change schedules for the applicable contaminants. It also simplifies employee training.

In addition, savings may be realized in certain situations through substituting air-purifying respirators for supplied-air respirators. Real-time ESLIs may be used with certain contaminants that cannot be detected by sensory means, such as H2S and TDI - contaminants that prior to the change in OSHA's ruling required more expensive supplied-air respirators.

During annual fit testing, end-users often review their respiratory program. This may be an opportune time to integrate ESLI-equipped cartridges into the program. If a change in respirators is necessary, fit testing of the new respirator and employee training on proper use and maintenance are also required. Some respiratory manufacturers provide videos and other assistance in the training process.

For respirators with ESLI-equipped cartridges, there is no need to develop and administer cartridge change schedules. The employee simply is instructed to change the cartridges when the colorimetric strip changes color.

Cost-effective method

The reason OSHA mandated objective methods, rather than subjective sensory means, for determining APR cartridge change times is apparent in the case of the employees who became desensitized to even the strong rotten-egg smell of hydrogen sulfide.

Either carefully determined cartridge change schedules or cartridges with readily visible ESLIs inside an air-purifying respirator cartridge offer objective means that meet OSHA-published standards on methods for determining APR cartridge change times. Because ESLI technology had not yet been developed when objective measures were initially required to be implemented, complicated and administratively burdensome cartridge change schedules were the primary objective method for determining change times.

Today, ESLIs are available for determining APR cartridge change times for selected acids, ammonia and TDI. This more accurate, less burdensome method will enable smaller companies with limited programs to more easily comply with OSHA regulations and will reduce administrative costs for already compliant companies. Use of air-purifying respirators with ESLIs is leading to more easily administered, more cost-effective respiratory protection programs.

SIDEBAR: ESLI benefits

  • Takes the guesswork out of estimating cartridge service life; eliminates the need for cartridge change schedules.
  • Provides cost savings from not having to discard cartridges before the end of their useful service life.
  • Gives added peace of mind to workers who have visible evidence of cartridge change times. They are assured that the cartridge is not being used beyond its normal service life.
  • Offers companies an easier way to stay in compliance with OSHA than depending on subjective "sensing" methods.

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