Priority Indicator #2: Percentage of Days with Good Air Quality

Contents: Background Information | Data Tables | Data Charts | Commentary | Download Data

Background Information

What Does This Indicator Mean?
Good air quality is a prerequisite for the health and well being of humans and ecosystems. This indicator measures the percentage of days per year in which the Air Quality Index (AQI) is in the good category, with a score between 0 and 50. In order for the AQI to be in this category, the amount of ozone, carbon monoxide, particulate matter, and other pollutants in the air must be below specific levels.  For each pollutant, this indicator is calculated as the total number of days with good air quality divided by the number of days monitored.

Other Risk Factors or Indicators to Consider
Ground level ozone (O3) is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources react chemically in the presence of sunlight.  Ozone at ground level is a harmful pollutant, and is of particular concern in the summer months.  Ozone also occurs naturally in the Earth's upper atmosphere, where it shields us from the sun’s harmful ultraviolet rays. This beneficial ozone is gradually being destroyed by artificial chemicals.

“Particulate matter” (PM) includes both solid particles and liquid droplets found in air. Particles less than 10 micrometers in diameter tend to pose the greatest health concern because they can be inhaled into and accumulate in the respiratory system. Particles less than 2.5 micrometers in diameter are referred to as “fine” particles. Sources of fine particles include all types of combustion (motor vehicles, power plants, wood burning, etc.) and some industrial processes. Particles with diameters between 2.5 and 10 micrometers are referred to as “coarse.” Sources of coarse particles include crushing or grinding operations, and dust from paved or unpaved roads.

Carbon monoxide (CO) is an odorless, colorless gas. It forms when the carbon in fuels does not completely burn. Vehicle exhaust contributes roughly 60 percent of all carbon monoxide emissions nationwide, and up to 95 percent in cities. Other sources include fuel combustion in industrial processes and natural sources such as wildfires. Carbon monoxide concentrations typically are highest during cold weather, because cold temperatures make combustion less complete and cause inversions that trap pollutants low to the ground.

These pollutants can irritate the respiratory system, reduce lung function, aggravate asthma, increase susceptibility to respiratory infections, inflame and damage the lining of the lungs, reduce the amount of oxygen in the bloodstream, cause chest pain, and increase the risk of premature death or admission to a hospital emergency room. For more information, visit http://www.epa.gov/airnow.

Limitations of the Data
Ozone, particulate matter, and carbon monoxide are currently the only pollutants included in this indicator.  Sulfur dioxide and nitrogen dioxide are not monitored in Marion and Polk Counties.  Also, air quality is only monitored in two locations in this area.  Carbon monoxide and particulate matter are measured at the intersection of Lancaster and Market Streets in Salem, and ground level ozone is measured at Cascade Junior High School in Turner.

Data Sources

Presentation and Description of Data

The table below shows the percentage of days with good air quality for ozone, particulate matter, and carbon monoxide.  It shows Marion County total rates only, since there are no monitoring sites in Polk County.

  1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
Ozone       97% 86% 98% 92% 97% 100% 99% 98% 92%
Particulate Matter (2.5)               86% 86% 89% 88% 91%
Carbon Monoxide 82% 65% 74% 87% 89% 94% 98% 92% 93% 98% 97% *100%

Source: Oregon Department of Environmental Quality data, compiled by Marion-Polk CHIP.  http://www.willamette.edu/publicpolicy/chip

*The condition for "good" has changed from <4.5ppm to <4.4ppm

The chart below shows the percentage of days in which the amount of ozone, particulate matter, and carbon monoxide in the air are at acceptable levels.  For ground level ozone, this level is less than 0.064 parts per million.  For carbon monoxide the standard is less than 4.5 parts per million, and for particulate matter it is less than 54 micrograms per cubic meter.  

Commentary

The amount of ground level ozone in the air was not monitored locally prior to 1995. Since that time, the percentage of healthy days (as measured by ozone levels) has fluctuated from year to year. Between 1998 and 2000 the percentage of healthy days increased to a full 100% in 2000. Unfortunately, since 2000 there has been a constant decline back down to 92% (the 1998 level).

The percentage of healthy days as measured by particulate matter 2.5 has continued get better, although not in a constant linear progression. Earlier data is currently being sought.

Levels of carbon monoxide were relatively high between 1992 and 1995 but have been decreasing in recent years.  The percentage of healthy days as measured by carbon monoxide levels increased from 1993 to 1998, decreased in 1999 and 2000, and increased again in 2001; carbon monoxide levels remained relatively stable through 2002.  Early in the decade, carbon monoxide levels caused more unhealthy days in Marion County than did levels of the other pollutants. More recently, however, carbon monoxide levels have decreased and become more consistent with ozone and particulate levels.

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This page last updated 2 August 2004