Nicotine is an alkaloid, a naturally occurring compound primarily found in the tobacco plant. As a potent neuro-active substance, it affects the central and peripheral nervous systems. Nicotine can be readily absorbed transdermally, meaning it enters the bloodstream directly through the skin. This ability is why products like nicotine patches were developed for use in nicotine replacement therapy.
The Mechanism of Transdermal Absorption
The skin’s outermost layer, the stratum corneum, serves as the body’s primary protective barrier against foreign substances. Most chemicals struggle to penetrate this lipid-rich, brick-and-mortar structure to reach the underlying blood vessels. Nicotine, however, is one of the few compounds capable of passive diffusion across this layer.
Nicotine’s specific chemical structure grants it an amphiphilic nature. It is a relatively small molecule that possesses both water-soluble and lipid-soluble properties. The lipid-solubility allows the compound to dissolve through the fatty cell membranes of the epidermis and move toward the deeper layers of the skin. Nicotine is absorbed most efficiently when it is in its uncharged, or “free base,” form, which bypasses cell barriers more easily than its salt form.
Once nicotine successfully crosses the stratum corneum, it enters the dermis, where it is absorbed into the capillary network and distributed systemically. Nicotine administered through the skin has high bioavailability because it bypasses the initial metabolic breakdown that occurs when a substance is swallowed. This steady, sustained absorption rate makes transdermal delivery an effective therapeutic method, as it avoids the sharp peaks seen with inhalation.
Practical Scenarios for Accidental Exposure
The most significant risk for acute transdermal exposure comes from concentrated liquid nicotine used in vaping products, such as e-liquids and refill bottles. These liquids contain nicotine in much higher concentrations than traditional tobacco products. Spilling even a small amount of a high-concentration solution on the skin can lead to rapid and substantial absorption, posing a serious poisoning risk.
Nicotine replacement therapy (NRT) patches, while designed for controlled release, also present an accidental exposure scenario, especially for children. Children may inadvertently come into contact with used patches that still contain residual nicotine or accidentally wear a patch for a prolonged period. Misuse by adults, such as simultaneously wearing multiple patches, can also lead to an overdose from excessive transdermal absorption.
Occupational exposure represents another practical route for transdermal absorption, primarily affecting agricultural workers. Individuals who harvest or handle raw, wet tobacco leaves can absorb significant amounts of nicotine through their skin, a condition historically known as “green-tobacco sickness.” The moisture on the leaves allows the nicotine to be transferred and absorbed, leading to symptoms of toxicity.
Signs of Nicotine Overexposure
Overexposure to nicotine through the skin can result in acute nicotine toxicity. Mild symptoms may include gastrointestinal distress, such as nausea, vomiting, and abdominal pain. Other early signs are neurological and cardiovascular, including dizziness, headache, sweating, and an increased heart rate, known as tachycardia.
If the absorbed dose is large, symptoms can progress to a more severe stage. These signs can include confusion, muscle tremors, seizures, and a drop in blood pressure. In the most severe cases of poisoning, central nervous system depression can lead to shallow breathing, respiratory failure, or coma.
If accidental skin exposure to a concentrated nicotine product occurs, immediate action is necessary to minimize absorption. The area of contact should be thoroughly washed with soap and water for an extended period. It is advisable to contact Poison Control or seek emergency medical attention, especially if symptoms like vomiting, dizziness, or confusion begin to develop.