CHAPTER 2-107: METHYLENE CHLORIDE

INTRODUCTION

Methylene chloride (dichloromethane, DCM) is a volatile, colorless liquid with a chloroform-like odor. Even though DCM is thought to be one of the least toxic chlorinated hydrocarbons, it can cause substantial toxic effects and mortality when used improperly. It has a wide variety of industrial uses, many of which are based on its solvent properties, including paint stripping, bathtub refinishing, pharmaceutical manufacturing, metal cleaning and degreasing, adhesives, film base production, agricultural fumigation, and plastics manufacturing. Methylene chloride is metabolized to carbon monoxide in vivo and may produce phosgene, chlorine, or hydrogen chloride upon combustion.

MECHANISM OF TOXICITY

1. Solvent effects. Like other hydrocarbons, DCM is an irritant to mucous membranes, defats the skin epithelium, and may sensitize the myocardium to the dysrhythmogenic effects of catecholamines.

2. Anesthetic effects. Like other halogenated hydrocarbons, DCM can cause CNS depression ranging from mild sedation to coma.

3. Carbon monoxide (CO) is generated in vivo during metabolism by mixed-function oxidases (CYP2E1) in the liver. Elevated carboxyhemoglobin (CO-Hgb) levels may be delayed and prolonged. CO-Hgb levels associated with DCM are usually lower than severe exogenous exposures to CO, but a level as high as 50% has been reported (see also "Carbon Monoxide").

4. Methylene chloride is a suspected human carcinogen (IARC Group 2B).

TOXIC DOSE

Toxicity may occur after inhalation or ingestion.

1. Inhalation. Inhalation toxicity typically occurs when DCM is used in poorly ventilated, enclosed areas. The permissible exposure limit (PEL) is 25 ppm as an 8-hour time-weighted average. The ACGIH workplace threshold limit value (TLV-TWA) is 50 ppm (174 mg/m³) for an 8-hour shift, which may result in a CO-Hgb level of 3–4%. The short-term exposure limit (STEL) is 125 ppm. The air level considered immediately dangerous to life or health (IDLH) is 2, 300 ppm. The odor threshold is about 100–200 ppm.

2. Ingestion. The acute oral toxic dose is approximately 0.5–5 mL/kg.

CLINICAL PRESENTATION

1. Inhalation is the most common route of exposure and may cause irritation of mucous membranes, upper airway and skin, nausea, vomiting, tachypnea, sweating, and headache. Ocular exposure can cause conjunctival irritation. Severe exposure may lead to pulmonary edema or hemorrhage, cardiac dysrhythmias, and CNS and respiratory depression.

2. Ingestion can cause corrosive injury to the GI tract and systemic intoxication. Renal and hepatic injury and pancreatitis have been reported.

3. Dermal exposure can cause dermatitis or chemical burns, and systemic symptoms can result from skin absorption.

4. Chronic exposure can cause bone marrow, hepatic, and renal toxicity. Methylene chloride is a known animal and a suspected human carcinogen (IARC Group 2B).

DIAGNOSIS

Is based on a history of exposure and clinical presentation.

1. Specific levels

   1. Carboxyhemoglobin levels should be obtained serially as CO-Hgb levels may have a delayed peak and prolonged elimination.

   2. Expired air and blood ...