As most of you know, OSHA establishes Permissible Exposure Limits (PELs) in order to protect workers who have the possibility of being exposed to harmful chemicals. PELs are usually expressed as ceiling limits or time-weighted averages for the short-term and long-term – 15 minutes and 8 hours respectively. While knowing these limits are important for anyone that could potentially come in contact with the chemicals, it is also both interesting and helpful to understand where the numbers themselves came from.
In 1954, three scientists performed a series of experiments at the Army Chemical Center in Maryland intended to shed more light on the toxicity of hydrogen peroxide (H2O2) vapor. The study, “Inhalation Toxicity of Ninety Per Cent Hydrogen Peroxide Vapor: Acute, Sub acute, and Chronic Exposures of Laboratory Animals”, involved exposing animals to various levels of H2O2 and observing the health effects. The paper they produced is considered classic research on H2O2 exposure and was the basis for the American Conference of Government and Industrial Hygienists’ (ACGIH’s) threshold limit value (TLV) of 1 ppm for H2O2. OSHA was created in 1970 and it adopted the ACGIH TLV’s as the OSHA PELs a couple of years later.
In the first experiment, rats were exposed to a very high (2880 ppm) concentration of H2O2 for 8 hours. While none of the rats died, they all showed severe congestion of lungs and trachea immediately following exposure. Within 3 days, pulmonary edema occurred and within 14 days, the majority of lungs showed many areas of alveolar emphysema as well as severe congestion. What the scientists learned was that while there was initially minor irritation, more severe symptoms developed later. There were similar results when the concentration of H2O2 was reduced to 243 – 308ppm for 8 hours.
In the second experiment, rats and mice were exposed to about 67ppm of H2O2 vapor for 6 hours a day, 5 days a week, for 6 weeks. Results included profuse nasal discharge after 2 weeks of exposure, hair loss around the nose in the 5th week (most likely due to animals scratching to relieve irritation), high rates of death among mice (80% after 18 exposures), and some congestion in the lungs.
The third experiment, however, was perhaps the most telling. In this series, dogs were exposed to 7ppm of H2O2 for six hours a day, five days a week, for 6 months. It was found that while there were no toxicity signs for the first 23 weeks, long-term exposure caused baldness, hair bleaching, and permanent lung damage, as well as general irritation such as sneezing and lacrimation.
Because of these results, the scientists concluded that long term exposure to even low concentrations of ~7ppm or more will produce impermanent lung damage in animals. Furthermore, one of the most important results from the study is that health effects resulting from exposure often do not appear immediately, and often present themselves days, or weeks later. From an occupational health perspective it should be noted that hydrogen peroxide has almost no odor and so is imperceptible until well above the OSHA PEL of 1 ppm. Therefore, continuous monitoring of the workplace to prevent exposures before they occur is recommended for all facilities using high concentrations of H2O2 where there is the potential for employee exposure.