Pulse oximetry, or SpO2, is a vital sign used across healthcare systems to gauge how much oxygen blood is carrying as a percentage of the maximum it could carry. Recent research has suggested that current SpO2 monitors may inaccurately report high SpO2 in patients with darker skin tones when the actual oxygenation is at unsafe, low levels. Additionally, this new research suggests as the SpO2 levels decrease, the risk of occult hypoxia rises. The investigators hypothesize melanin interferes with the pulse oximetry accuracy. Investigators will use spectrophotometry to measure melanin indices and other variables to test this hypothesis.

Inclusion criteria:

1. >18 years old
2. Any gender
3. Any ethnic background
4. Admitted to UC San Diego La Jolla or Hillcrest campus ICU
5. Mechanically ventilated
6. Has a clinical ABG ordered by the critical care team responsible for clinical care

Exclusion Criteria a. <18 years old b. Medical history of or acute diagnosis of sickle cell disease c. Normally protected individuals under 45 CFR 46 (pregnant individuals, prisoners, and children)

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Critically ill patients on mechanical ventilation.