Pathways of Exposure

The pathways of exposure to lead for humans and animals include ingestion, inhalation and dermal (i.e., skin) contact. Lead is common in the environment and exposure mainly comes from industrial lead products like leaded paint, gasoline and pesticides and the legacy remnants of those products that are found in soil, dust and water.  These highly soluble forms of compounded lead can accumulate in an organism and can cause lead poisoning (toxicity).

Ingestion

Lead ingestion is the primary source of exposure and can cause two different types of lead poisoning, either acute or chronic.  Acute lead poisoning may result from either a single ingestion of a soluble form of lead, or by multiple episodes within a short time period, causing a large spike in an organism’s blood-lead levels.  The organism’s kidney and liver naturally filter and absorb some of the blood-lead, resulting in higher concentrations of lead within those organs.A real world example of acute lead poisoning is typically associated with cattle.  Cattle are exposed to many sources of highly soluble forms of compounded lead in the environment, such as discarded lead sulfate containing batteries and old wood with peeling lead paint. After a cow inadvertently ingests a large amount of soluble forms of lead, blood-lead levels spike. The cow’s liver and kidneys attempt to filter out the lead, but ultimately the ingestion may lead to its death. The dead cow now lies in the field. Its organs contain a very high concentration of lead that has already been metabolized and is extremely soluble. Subsequently, a scavenger like vultures, California condors, ravens, bears or wolves may feed on the carcass before a rancher finds and removes the cow from the field. Consequently, the scavenger ingests significant levels of lead, and this soluble form of lead can easily cause lead poisoning in that animal.

Chronic lead poisoning can occur by ingesting sub-lethal amounts of soluble lead over a long period of time. When an organism consumes a lower amount of such lead, it can slowly sequester the lead from the blood and organs into skeletal tissue, effectively “locking” the lead away so it does not remain in the blood supply and harm the organism. Because chronic lead poisoning causes similar non-specific symptoms such as peripheral nerve damage, gastrointestinal disorders and diminished coordination, it can be very difficult to diagnose without accurate blood testing.

When an organism ingests metallic lead (insoluble lead), the lead resists breakdown under normal digestive conditions in the body and passes out of the organism relatively unchanged.  When an organism ingests soluble forms of compounded leadfound in industrial products, such as leaded paint, gasoline and pesticides, the lead is readily absorbed in the body and increases blood-lead levels significantly.

Inhalation

Inhalation of lead is mainly attributed to dust particles containing compounded industrial lead from leaded paint, gasoline and pesticides. These highly soluble legacy lead compounds remain in large quantities in the environment, and are deposited into the lungs when inhaled and are directly transmitted to the bloodstream.

Inhalation of lead can lead to elevated blood-lead levels in both humans and animals. Indeed, the California condor has been found to suffer from high rates of anthracosilicosis, which is a form of lung condition associated with the inhalation of dust particles.  Because the condors inhale so much dust in their daily activities, the associated residual lead compounds in the soil may significantly contribute to their elevated blood-lead levels.

Topical

Dermal exposure to (aka skin contact with) lead is the least likely pathway of exposure to both humans and animals. Industrial lead compounds found in leaded gasoline (tetraethyl lead), which is still used in certain aviation gasolines, is easily absorbed through the skin.  Once absorbed, the highly soluble form of lead is transmitted to the bloodstream and can cause elevated blood-lead levels.

Whether ingested, inhaled or absorbed through the skin, the effect of these lead substances on an organism depends upon the chemical properties of the substance, and how the organism retains, processes, and eliminates the substance. The time of retention of a substance is an important factor in how much of the substance ultimately is bioavailable to affect an organism. Also, whether a substance is retained in an organism’s body, and at what concentration, are likewise important factors in determining the potential effect of the lead substance on an organism.

When applied to the discussion of hunting with lead ammunition, the impact of ingesting insoluble metallic lead, in comparison to the impact of ingesting the highly soluble forms of industrial lead compounds, the effect on the organism is starkly contrasted.