Occupational Medicine
A Basic Guide

03: Medical Surveillance

Biological Monitoring
George Grimes, MD, MPH, FACOEM

Biological monitoring in the occupational setting is used to determine levels of exposure and/or health effects for workplace hazards. Most commonly, biological monitoring is performed during routine medical surveillance examinations. However, it may also be used in response to a specific event resulting in high exposures to a known hazard. Its primary goal is to identify substances or metabolites in the body or subclinical health effects before they cause negative health outcomes.


Biological monitoring involves both direct testing of substance levels in the body and indirect testing to determine the magnitude of an exposure’s effect, without directly measuring the substance or metabolite itself. An example of direct testing is checking for cadmium exposure through blood cadmium levels. An example of indirect testing is the beta-2 microglobulin urine test for cadmium, which tests for kidney damage as a result of chronic cadmium exposure.


Biological monitoring can be a required component, or a useful adjunct, to a medical surveillance program. To monitor workers’ exposure, medical surveillance programs generally rely on biomarkers of both exposure and effect. Biomarkers of exposure provide a measure of body burden for a chemical, whereas biomarkers of effect measure the physiologic responses in the body from a hazard. Biological tests not only help physicians confirm exposures or diagnoses but also assist industrial hygienists in determining which workers are at risk and the effectiveness of workplace controls.


Biological monitoring can be performed for the full spectrum of occupational hazards, from physical hazards such as noise to more common chemical hazards such as lead. OSHA requires biomonitoring with mandated surveillance for three chemicals: benzene, cadmium, and lead. These chemicals span a wide range of industries, from the oil and gas industry to military defense to construction. OSHA mandates that biological monitoring for these chemicals occur once the hazard has been identified in air concentrations in excess of occupational exposure limits. For example, if an industrial hygienist performing a hazard assessment determines that an employee is exposed to air concentrations of cadmium at or above the action level (2.5 µg/m3 over an 8-hour time-weighted average) for 30 days or more in a 12-month period, the employer is obligated to enroll the employee (and employees in a similar exposure group) into a medical surveillance program. This program includes biomonitoring, relying on urine cadmium, blood cadmium, and urinary beta-2 microglobulin to assist in determining whether workers are experiencing both hazardous exposure to and effects from cadmium. For lead, OSHA mandates that employees receive blood lead testing once air levels exceed the action level (30 µg/m3 over an 8-hour time-weighted average) in the workplace. However, recent evidence has shown that blood lead levels may reach harmful levels at air concentrations below occupational exposure limits. Thus, using blood, rather than air, lead levels to drive workplace controls can help prevent overexposures and subsequent health effects.


In addition to those chemicals regulated by OSHA-mandated medical surveillance programs, many others are associated with biomarkers that help the provider determine whether hazardous exposures or effects have occurred. The American Conference of Governmental Industrial Hygienists (ACGIH) publishes biological exposure indices (BEI®)—guidelines to assist in controlling workplace hazards that list concentrations in the body for substances and metabolites that correlate with air levels of the substance. If chemical concentration levels exceed a BEI, they may indicate workers’ overexposure to a toxic chemical. With sustained overexposure, adverse health effects may develop. BEIs should always be interpreted and applied by an individual trained in industrial hygiene and/or occupational medicine. The BEI Guidelines are available for purchase.


Establishing a Biological Monitoring Program

Specific criteria must be met before an employer initiates a biological monitoring program. First, the chemical must be present in a medium suitable for sampling, such as blood, urine, or exhaled air. The collection method should be discussed and found acceptable to those individuals who are being tested. For example, if a workforce generally refuses blood testing because it is seen as invasive, urine testing may be considered a more acceptable alternative. Analyzing samples should be practical and produce valid, reproducible results. Furthermore, samples should be collected in a manner in which they are relevant to the work being done and can be interpreted meaningfully. Perhaps most important, the program should specify from the start an action plan to respond to aberrant results. If all of these criteria are met, and the sampling will do the workforce more good than harm, then a biological monitoring program can be established.


Proper timing of sample collection is important: those samples with short half-lives may be sampled during or at the end of work shift; whereas, samples with half-lives of years do not require immediate collection. Sample timing, collection methods, and media collected are all essential factors to consider when collecting samples.


Routes of Exposure

Environmental monitoring assesses chemical levels in the air in the workplace. Controls, such as respirators, can prevent airborne chemicals from entering a worker’s body. High air-chemical levels detected through personal-breathing-zone monitoring indicate overexposure, but adequate personal protective equipment is intended to interrupt the exposure pathway and prevent the worker from experiencing harmful health effects. If respirators appear adequate, yet workers continue to show elevated toxin levels on biological monitoring, other exposure routes must be considered. This issue is particularly important for chemicals that are readily absorbed through the skin. Additionally, chemicals that can contaminate the workplace, such as lead, may be ingested, resulting in a high body burden even when airborne levels appear negligible.


Reviewing providers must have a full understanding of the reasons for testing and the health effects of a chemical. All abnormal results on biological monitoring should lead to a thorough review of the worker’s health and the worker’s practices, procedures, and working conditions. However, actions taken as a result of biological monitoring should rely on multiple measurements rather than a single test. Workers should be counseled to share these results with their personal physicians, to ensure that primary care providers are aware of any findings discovered.


Biological monitoring must be performed at the expense of the employer, not the employee. The results should not affect the relationship between the employee and the employer, nor should it affect the employee’s pay. If an employee has elevated levels on biological testing and must be removed from the work environment, he or she must retain his or her pay and should not be penalized in any way. The length of medical removal depends on such factors as the time it takes to eliminate a chemical, the health effects experienced by the employee, or OSHA-mandated medical removal periods.