This code system http://hl7.org/fhir/allergy-intolerance-criticality defines the following codes:

CodeDisplayDefinition
lowLow RiskWorst case result of a future exposure is not assessed to be life-threatening or having high potential for organ system failure.
highHigh RiskWorst case result of a future exposure is assessed to be life-threatening or having high potential for organ system failure.
unable-to-assessUnable to Assess RiskUnable to assess the worst case result of a future exposure.