unhealthful day

Any day in which the air quality for any of the 5 major ambient air pollutants—carbon monoxide, nitrogen dioxide, ozone, particulate matter, sulphur dioxide—exceeds an established air quality standard

unhealthful day

Public health Any day in which the air quality for any of the 5 ambient air pollutants–carbon monoxide, nitrogen dioxide, ozone, particulate matter, sulfur dioxide exceeds an established air quality standard. See Pollution Standards Index. Cf Unhealthy day.
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Health-related air quality was described in terms of the number of good days, moderate days, unhealthful days, very unhealthful days, and hazardous days.