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Adapted from: do Nascimento et al., 2008 [2]
| Methemoglobin levels | Signs and symptoms |
| Up to <3% | None |
| 3-15% | Frequently none grayish skin |
| 15-30% | Cyanosis chocolate brown blood |
| 30-50% | Dyspnea, headache, fatigue, weakness, dizziness, syncope |
| 50-70% | Dyspnea, headache, fatigue, weakness, dizziness, syncope |
| >70% | Death |
