Visual guide: How to read the air monitoring results charts

Note: Two charts appear in each report — a West Seattle and South Park.  Only a South Park chart is used here.

What the air monitoring results mean

The summary shows concentrations of nitrogen oxide (NO) and sulfur dioxide (SO2) measured in parts per billion (ppb), typically up to 250 ppb.

Chemical concentrations, odor thresholds and exposure limits – The NO and SO2 concentrations detected at both the South Park and West Seattle Reservoir monitoring sites are far below the odor thresholds and exposure concentration limits established by the Occupational Safety & Health Administration and National Institute for Occupational Safety and Health (NIOSH).

An odor threshold, defined by EPA, is the minimum odor of a water or air sample that can just be detected after successive dilutions with odorless water. The human nose can often detect a chemical odor far below the threshold and can be more sensitive than today’s monitoring technologies. Odor threshold levels for NO and SO2 begin at approximately 300ppb. An exposure concentration, defined by EPA, is the concentration of a chemical or other pollutant representing a health threat in a given environment. NIOSH lists SO2 to become an Immediate Danger to Life or Health (IDLH) at 100,000 ppb; NO becomes an IDLH at 20,000 ppb.

*The wind direction noted in the South Park summary is measured by monitors at Duwamish, just a few miles northeast sharing similar topography. The wind direction noted in the West Seattle Reservoir summary is measured by monitors at Beacon Hill, the ridge to the east sharing similar topography.