When we think about exposures to chemicals causing adverse effects on human health, there is a tendency to view this as a product of modern industrial societies. To some extent this is true – there are certainly potentially hazardous chemicals we are exposed to as a result of our lifestyles, such as volatile chemicals in paints and solvents, the gasoline used to fuel our vehicles, and products of the combustion of tobacco. Most of the instances where we can clearly associate an adverse health effect with a particular chemical exposure are from workers in factories and chemical plants.
However, a new paper by Sebastian Wärmländer and colleagues examines a much older case of exposure to harmful chemicals – specifically aboriginal populations in California starting around 10,000 years ago exposed to polycylic aromatic hydrocarbons (PAH).
The Chumash Indians in the Channel Islands in California used naturally occurring bitumen for a variety of purposes, including as a sealant for water-bottle baskets and later canoes. Use increased over time, resulting in expected increased exposure to bitumen, including direct contact with the bitumen, inhalation of fumes, and ingestion of contaminated water and seafood.
PAH, which are a major component of bitumen, are known to have a variety of toxic effects in humans, including cancer; PAH are likely responsible for most of the carcinogenicity of tobacco smoke and other combustion products. However, the effect looked at here is an association with reduced stature and head circumference in babies whose mothers were exposed during pregnancy.
Since effects on lifespan and cancer cannot be readily observed from skeletal remains, the authors looked at stature and head circumference. What they found was that, at a time in the history of the Chumash Indians when stature should have been increasing due to reduced inbreeding, stature and head circumference were decreasing, and that this decrease was correlated with increased PAH exposure. They also found some other evidence of decreasing health, such as skeletal indicators of disease.
Obviously proving cause and effect in a case such as this is difficult, and the authors acknowledge that they can’t do that with the information available. However, they do demonstrate a plausible relationship. PAH from bitumen are known to be toxic at environmentally relevant concentrations, and the use patterns observed in the Chumash Indians would likely have resulted in higher exposure than most people would get today due to our increased awareness of chemical hazards and occupational health and safety requirements.
One minor point in the paper annoyed me; the paper indicated that PAH accumulate along the food chain, and as a result the highest PAH levels may be found in higher trophic-level predators such as sharks and humans. While from their chemical properties alone, this might be expected, in fact PAH don’t bioaccumulate at higher trophic levels. Higher organisms, including mammals, metabolize PAH fairly rapidly. The metabolites themselves may be toxic and less is known about their fate, although they are likely metabolized further and excreted, but PAH themselves do not generally accumulate in higher trophic levels. This doesn’t really affect the conclusions of the paper, however.
Wärmländer, S., Sholts, S., Erlandson, J., Gjerdrum, T., & Westerholm, R. (2011). Could the Health Decline of Prehistoric California Indians be Related to Exposure to Polycyclic Aromatic Hydrocarbons (PAHs) from Natural Bitumen? Environmental Health Perspectives DOI: 10.1289/ehp.1103478