Sources of carbon monoxide include faulty furnaces, inadequate ventilation of heat sources, and engine exhaust exposure. Symptoms of carbon monoxide poisoning are non-specific. Mild exposure can cause headaches, myalgia, dizziness or neuropsychological disturbances. Severe exposure to carbon monoxide can lead to confusion, loss of consciousness, or death. Patients with subclinical exposure may be found to have carbon monoxide poisoning only after an acute event or by chance. In physiological amounts, endogenous carbon monoxide acts as a neurotransmitter. At low concentrations, carbon monoxide may favorably modulate inflammatory responses, apoptosis and cell proliferation, and upregulate mitochondrial biosynthesis. As carbon monoxide exposure increases, it can cause toxicity. Carbon monoxide causes hypoxia by forming carboxyhemoglobin and shifting the oxygenated hemoglobin dissociation curve to the left. Carbon monoxide has an affinity for hemoglobin that is more than 200 times greater than that of oxygen and can lead to carboxyhemoglobin formation at low concentrations. Carbon monoxide increases cytosolic hemoglobin levels, leading to oxidative stress and binding to platelet hemoglobin and cytochrome c oxidase, interrupting cellular respiration and causing the production of reactive oxygen species, leading to neuronal necrosis and apoptosis. Impaired cellular respiration causes a stress response that leads to neurological and cardiac protection or injury depending on the dose of carbon monoxide. Carbon monoxide exposure also causes inflammation through multiple pathways that are independent of the hypoxic pathway and lead to neurological and cardiac damage. With chronic subacute exposures to carbon monoxide lasting more than 24 hours, symptoms usually occur intermittently and may persist for weeks or even years. Symptoms of chronic poisoning may differ from those of acute poisoning and may include chronic fatigue, emotional problems and disturbances, memory deficits, difficulty working, sleep disturbances, vertigo, neuropathy, sensory abnormalities, recurrent infections, erythrocytosis, abdominal pain and diarrhea.