Biological Monitoring in Occupational Hygiene: When Urine and Blood Tests Are Required

Air monitoring measures what is in the air a worker breathes. Biological monitoring (BM) — sometimes called biomonitoring or biological exposure monitoring — measures what is actually in the worker’s body: the chemical itself, or a metabolite, in urine, blood, exhaled breath or other biological media. This is a fundamentally different and often more informative measure of total exposure — capturing all routes of entry simultaneously, including dermal absorption and incidental ingestion that air sampling completely misses.

Why Biological Monitoring Matters

For several important occupational chemicals, dermal absorption is a significant exposure route that air monitoring alone cannot quantify:

• Organophosphate pesticides: Skin absorption from handling and spraying is major. Blood or urinary cholinesterase activity is the primary exposure marker
• Glycol ethers (2-methoxyethanol, 2-ethoxyethanol): Significant dermal absorption from surface coating applications — urinary alkoxyacetic acid metabolites detect this
• Lead: Ingestion from hand-to-mouth contact in lead-contaminated environments can exceed inhalation exposure. Blood lead is the definitive exposure measure
• Mercury (inorganic): Both inhalation and dermal routes contribute to urine mercury levels
• Aniline and some aromatic amines: Readily absorbed through skin; urinary metabolites confirm total exposure

In these situations, an employer who relies solely on air monitoring to demonstrate compliance may be missing a significant portion of total worker exposure.

Biological Exposure Indices (BEIs)

The ACGIH publishes Biological Exposure Indices (BEIs) as the primary reference values for biological monitoring in occupational settings. BEIs represent the biological levels expected in a reference healthy population exposed at the ACGIH TLV (threshold limit value) for the corresponding substance. They are not “safe” thresholds — they are reference points for comparison, not health-based limits per se.

Australian practice uses ACGIH BEIs as the primary reference for biological monitoring interpretation, given that Safe Work Australia’s own WES document includes limited biological monitoring reference values. Where the WES differs from the ACGIH TLV, the BEI may need contextual adjustment — an occupational physician’s guidance is warranted for substances where this distinction is significant.

Sampling Timing: When to Collect the Sample

The timing of biological monitoring sample collection relative to the work shift is critical — collect at the wrong time and the result is meaningless or misleading. Three main timing categories:

• During the shift: For substances with short biological half-lives where concentrations peak during exposure (e.g., carbon monoxide — exhaled CO sampled towards end of exposure period)
• End of shift (EOS): For substances that reflect acute cumulative exposure within the shift (e.g., urinary mandelic acid for styrene, urinary toluic acid for toluene)
• End of shift at end of week (EOSW): For substances that accumulate over the work week due to longer biological half-lives (e.g., blood lead, urinary mercury — these reflect ongoing cumulative exposure)
• Before next shift (pre-shift): For persistent substances where background level is the concern (e.g., urinary cadmium — mainly reflects lifetime body burden)

The BEI documentation specifies the required timing for each substance. Collecting a urine sample at the wrong time point — beginning of shift for an EOS determinant — will produce a falsely low result and provide false assurance of compliance.

Key Substances and Their Biological Monitoring Parameters

Interpretation Pitfalls

• Creatinine adjustment for urinary determinants assumes normal renal function and hydration — results in very dilute (creatinine <0.3 g/L) or very concentrated samples may be unreliable
• Background non-occupational exposure (smoking for CO, diet for certain metals) must be accounted for
• Individual pharmacokinetic variability — some workers metabolise substances faster or slower than average, producing results outside expected ranges at the same exposure
• Elevated results require occupational physician review before conclusions are drawn — the hygienist’s role is exposure assessment, not medical diagnosis