Tracing PBB Homologs in Michigan's Exposed
Imagine a simple mix-up at a chemical plant that would end up exposing millions of people to a toxic substance, leaving a biological signature in their bodies that scientists would study for decades.
This isn't science fiction—it's exactly what happened in Michigan in the 1970s when polybrominated biphenyls (PBBs), flame retardants meant for plastics, were accidentally incorporated into animal feed. This agricultural disaster contaminated Michigan's food supply and created a natural laboratory for studying how chemicals persist in the human body and how different exposure pathways leave distinct biological footprints.
By examining the subtle chemical patterns in the blood of exposed dairy farmers and chemical workers, scientists have uncovered fascinating insights about environmental exposures that continue to inform public health today.
The Michigan PBB incident created a unique opportunity to study how different exposure routes (inhalation vs. ingestion) affect the distribution of chemical components in the human body.
Polybrominated biphenyls are a class of industrial chemicals developed as flame retardants for use in plastics, textiles, and electrical appliances. Their chemical structure consists of two connected benzene rings (biphenyl) with bromine atoms attached at various positions.
This basic architecture can be configured in 209 different arrangements known as congeners, each with distinct biological properties and toxicity profiles 4 .
The commercial PBB mixture involved in the Michigan incident contained approximately 22 different PBB congeners:
A shipment of FireMaster FF-1 was mistakenly sent to a feed mill in place of NutriMaster, a nutritional supplement for livestock. Both products were fine white powders stored in nearly identical bags with minimal labeling distinction 4 .
Farmers began observing alarming health effects in their livestock: cows stopped eating, milk production plummeted, animals developed abnormal hoof growths and skin conditions, and many died prematurely.
After a full year since the initial mistake, officials identified PBB as the cause. By then, an estimated 6.5 million Michigan residents had been exposed through consumption of contaminated animal products 4 .
Cattle destroyed
Pigs destroyed
Chickens destroyed
In November 1976, researchers from the Environmental Sciences Laboratory of the Mount Sinai School of Medicine initiated a comprehensive health examination of 1,029 Michigan residents potentially affected by PBB exposure.
This cohort included 55 Michigan Chemical Corporation employees and 237 farming families from quarantined farms, creating a unique opportunity to compare how different exposure pathways might influence the biological processing of these chemicals 1 .
Gas Chromatography-Mass Spectrometry (GC-MS) was used to separate complex chemical mixtures and identify individual components based on their molecular weight and structure 1 .
This technique allowed researchers to distinguish between the various PBB congeners present in the serum samples.
The serum analysis provided compelling evidence that the human body doesn't process all PBB congeners equally, and that exposure pathways matter significantly in determining which chemicals accumulate in our systems.
The discovery of distinct homolog patterns suggested that certain bromine arrangements might be more readily metabolized or eliminated depending on how they enter the body 1 .
The exposure incident occurred in 1973, yet by 1976—three years later—detectable levels of PBBs remained in the blood of exposed individuals.
This demonstrated that lipophilic (fat-loving) chemicals like PBBs can accumulate in body fat and resist elimination.
| Homolog Group | Chemical Workers | Farming Families | FireMaster FF-1 |
|---|---|---|---|
| Pentabromobiphenyls | Different relative concentrations | Different relative concentrations | Present as minor components |
| Hexabromobiphenyls | Multiple peaks with distinct ratios | Multiple peaks with distinct ratios | ~60% of mixture |
| Heptabromobiphenyls | Different relative concentrations | Different relative concentrations | Present as minor components |
| Congener | Bromine Substitution Pattern | Approximate Percentage in Mixture | Notes |
|---|---|---|---|
| PBB-153 | 2,2′,4,4′,5,5′ | ~60% | Primary component, hexabromobiphenyl |
| PBB-180 | 2,2′,3,4,4′,5,5′ | Significant minority | Heptabromobiphenyl |
| Various | Multiple patterns | Minor components | Pentabromobiphenyls and additional hexa-/hepta-bromobiphenyls |
The initial serum homolog study established a foundation for what would become one of the most extensive longitudinal investigations of chemical exposure in public health history.
The Michigan PBB Registry was established to track the health outcomes of exposed individuals, eventually enrolling approximately 5,000 participants initially, with many of their children subsequently added to the cohort 4 .
In 2019, an epigenome-wide association study revealed that PBB exposure was associated with differences in DNA methylation at 1,890 different sites in the genome 2 .
These epigenetic changes were enriched in regions related to immune function and endocrine-related autoimmune diseases.
| Tool/Reagent | Primary Function | Significance in PBB Research |
|---|---|---|
| Gas Chromatography-Mass Spectrometry (GC-MS) | Separation and identification of chemical compounds | Enabled detection and quantification of specific PBB congeners in complex biological samples like blood serum 1 |
| FireMaster FF-1 Reference Standard | Chemical reference material | Provided baseline for comparing congener patterns in environmental and biological samples against the original contaminant 1 |
| Serum Biobank | Biological sample preservation | Allowed for long-term study of exposed populations; enabled retrospective analyses as new questions emerged 1 2 |
| MethylationEPIC BeadChip | Epigenetic analysis | Used decades after exposure to identify DNA methylation changes associated with PBB levels in blood 2 |
| Toxicity Estimation Software Tool (TEST) | Computational prediction of chemical properties | Enabled estimation of bioconcentration factors and other environmental parameters for different PBB congeners 6 |