Describe the various exposure routes of toxicants in the human body. Discuss the mode of action of toxic substances.

### Various Exposure Routes of Toxicants in the Human Body  

Environmental toxicology focuses on understanding how toxicants interact with biological systems, particularly humans. The entry of toxic substances into the human body can occur through several primary exposure routes. Each of these routes affects the absorption, distribution, metabolism, and excretion (ADME) of toxicants in distinct ways.  

#### 1. Inhalation (Respiratory Route)  
One of the most significant and rapid routes of toxicant entry is through inhalation. Airborne toxicants, such as gases, vapors, aerosols, and particulate matter, enter the respiratory system and are absorbed primarily through the alveolar membrane in the lungs. Since the lungs have a vast surface area and an extensive capillary network, toxicants can quickly enter the bloodstream and reach systemic circulation.  

- Examples: Carbon monoxide, sulfur dioxide, nitrogen oxides, volatile organic compounds (VOCs), and industrial solvents.  
- Effects: Some inhaled toxicants cause direct respiratory irritation (e.g., ammonia), while others exert systemic effects, such as carbon monoxide binding to hemoglobin, reducing oxygen transport.  

#### 2. Ingestion (Oral Route)  
Toxicants can enter the body through ingestion of contaminated food, water, or soil. The gastrointestinal (GI) tract facilitates absorption, primarily in the small intestine due to its large surface area and high permeability. After absorption, toxicants undergo first-pass metabolism in the liver, where they may be detoxified or bioactivated.  

- Examples: Heavy metals (lead, mercury), pesticides (organophosphates), food additives, and mycotoxins.  
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