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Assessing Outdoor Air Near Schools

Sample Screening Levels

EPA developed individual “sample screening levels” for each key pollutant monitored at the school.  These values help EPA to identify any measurements which warrant closer attention, to confirm sample results, and to assess the potential for health concerns.  Sample results at or below the sample screening level are not of concern for risk of adverse health effects from continuous (24-hours-a-day, every day) short-term exposures, which might range from a day up to at least a couple of weeks (longer for some pollutants).  Individual sample results above the sample screening level do not mean that there is a risk to children and adults at the school.  Rather, sample results higher than these sample screening levels will be further considered in light of chemical-specific information on health effects, and with information concerning the sample collection and potential sources of the monitored pollutant, and also the pattern of levels across multiple samples within the monitoring period. 

In developing or identifying these screening levels, EPA gave priority to use of relevant and appropriate air standards* and EPA risk assessment guidance and precedents.  The Agency relied upon health effects information developed and assessed by EPA, the U.S. Agency for Toxic Substances and Disease Registry, California EPA and others.  This use is consistent with the way EPA has used this information for similar activities in the past.    

The individual sample screening levels represent exposure estimates that are unlikely to lead to a risk of adverse health effects for children or adults exposed all day, everyday, for periods of a couple of weeks (and, for some pollutants, up to a year).  Accordingly, the average of multiple measured concentrations over two weeks (or longer for some pollutants) that is at or below these levels is not of concern.  That is, an individual sample measurement, even, in some cases, multiple sample measurements, greater than the screening level does not imply an immediate health threat and should not be presumed to be cause for alarm.  Rather, findings of individual sample measurements above these screening levels receive additional attention to confirm sample results and to assess the potential for any health concerns.  In general, such results indicate the need for close attention to those pollutant results, as well as the need for assessment of longer-term data trends, which will be the focus of EPA’s analysis of the full dataset.

Consideration of sensitive and susceptible groups:  The screening levels developed by EPA draw from exposure and risk values that were derived by the organizations identified above using generally similar methods.  In all cases, the methods account for the potential for differences in sensitivity or susceptibility of different groups (e.g., asthmatics) or ages (e.g., young children) to a particular pollutant effects so that the resulting values are protective of these groups.  Further, in some cases (e.g., lead), the health effects information has led us to focus specifically on children as the sensitive population as the target in developing the level.

Metals: With the exception of lead, the sample screening levels for metals (arsenic, cobalt, manganese, nickel, antimony, beryllium, cadmium, chromium, mercury, and selenium) are based on inhalation exposures and are to be used in comparison to concentrations of metals in particles captured in a PM10 sample. Particles in these samples are generally 10 micrometers in diameter or smaller and can be inhaled and enter the lungs. In contrast, EPA established the National Ambient Air Quality Standards (NAAQS) for lead with attention to non-inhalation exposure pathways, such as incidental ingestion of dust from the air that can be picked up onto children’s hands. Larger particles such as those collected in total suspended particles (TSP) samples can play an important role in this type of exposure. Consequently, the NAAQS for lead, the sample screening level used in this initiative, is specific to lead in TSP samples.

* In the case of lead, the recently revised national ambient air quality standard is the sample screening level.  Please note that this level applies to a 3-month average of lead measurements.  Individual sample results above this level do not indicate a long-term risk of an adverse health effect. Results above the level do indicate the need for careful monitoring and the assessment of longer-term data trends.

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