Arsenic poisoning is a medical condition resulting from elevated levels of arsenic within the body. Arsenic exists naturally in the earth’s crust and can enter the human system through contaminated air, food, or water. While blood and urine tests confirm very recent exposure, they are limited because the body rapidly clears arsenic from these fluids. Hair analysis offers a non-invasive, historical record of past arsenic exposure. The physical structure of hair acts as a permanent repository, making it an invaluable tool for forensic and chronic exposure investigations.
How Arsenic Enters and Stays in Hair
Hair is a reliable biomarker due to the unique chemical structure of its main protein, keratin. Keratin is rich in the amino acid cysteine, which contains reactive sulfhydryl groups. Once inorganic arsenic enters the bloodstream, it is distributed throughout the body and exhibits a high affinity for these chemical groups.
This strong binding effectively locks the arsenic into the hair matrix while the hair is being formed beneath the scalp. As the hair grows out, this arsenic-keratin complex becomes structurally fixed, permanently recording the systemic concentration of the toxin. The concentration of arsenic measured in a hair strand directly correlates to the amount that was circulating in the body during the growth period.
Clinical Signs and Sources of Arsenic Poisoning
Symptoms vary depending on the dose and duration of exposure, falling into acute or chronic categories. Acute, high-level arsenic ingestion typically causes severe gastrointestinal distress, including vomiting, diarrhea, and intense abdominal pain. This rapid onset can lead to dehydration, neurological issues like encephalopathy, and cardiovascular shock.
Chronic, low-level exposure manifests years later through visible skin changes. These include hyperpigmentation (darkening skin patches) and hyperkeratosis (thickening of the skin on the palms and soles). Another indicator of chronic poisoning is the appearance of Mees’ lines, which are distinctive white bands that form horizontally across the fingernails.
Common sources of arsenic exposure worldwide include naturally contaminated groundwater, which is the largest threat to public health. Other sources involve certain industrial processes, such as mining and copper smelting. Historically, arsenic-based pesticides and wood preservatives were also significant sources.
Mapping Exposure Over Time
Hair analysis can map the timeline of arsenic exposure due to the predictable rate of hair growth, averaging approximately one centimeter per month. This consistent speed allows forensic toxicologists to segment a hair shaft into chronological sections.
For instance, a six-centimeter sample can be cut into six one-centimeter segments, each representing roughly one month of exposure history. By analyzing the arsenic concentration in each segment, analysts can differentiate between a single, high-dose event (a spike in one segment) and prolonged, low-level exposure (consistent, elevated concentrations). This segmentation creates a precise chronological record, revealing when the exposure occurred over the preceding months or years.
The Testing Process and Sample Collection
The testing process begins with careful sample collection to ensure the integrity of the results. Hair is typically cut close to the scalp, usually from the occipital region, to capture the most recent growth. A sample length of three to six centimeters is collected to provide a multi-month history of exposure. Before analysis, the sample must be washed using a solvent like acetone or a detergent solution to remove external contamination, such as dust or chemicals. This washing ensures that the measured arsenic was absorbed internally from the bloodstream, not merely deposited on the hair surface.
The samples are then prepared and analyzed using laboratory techniques, most commonly Inductively Coupled Plasma Mass Spectrometry (ICP-MS). This instrument vaporizes the sample and measures the precise mass-to-charge ratio of the arsenic ions. This process allows for the accurate quantification of trace amounts of the metalloid present in the hair structure.