| Methaemoglobin | Signs and symptoms |
| 0–3% | Normal levels, no symptoms up to 10% |
| 10–20% | Mild symptoms, cyanosis, chocolate-brown coloured blood |
| 20–50% | Dyspnoea, exercise intolerance, fatigue, headache, dizziness, confusion, tachicardia, syncope |
| >50% | Central nervous system depression, seizures, coma, diysrhytmias, myocardial ischemia, tachypnea, metabolic acidosis, death |