Screening Levels

Screening levels for metals in ambient (outdoor) air detected near WTC that are hazardous air pollutants have been set to protect against significantly increased risks of cancer and other long term health problems.

For carcinogenic compounds, the screening levels were set so that a person exposed to WTC-related contaminants would not exceed a one-in-ten thousand probability (1x10-4) of developing cancer. To be conservative, the screening level was based on a one year continuous exposure duration. This was considered the upper- bound estimate for the length of time a member of the public would be exposed to products of combustion from the fires and re-entrained dust from the the collapse and demolition activities. The exposure duration (ED) differs from the ED of 30 years which was used for setting health based benchmarks for indoor air (Setting Health-Based Benchmarks), which was was considered the upper bound estimate for exposure to re-entrained dust deposited on indoor surfaces during residency.

Because these screening levels assume continuous exposure for an extended duration, the average of the measured concentrations is more appropriate for evaluating risk than any single measurement. Consequently, individual sample results did not exceed a long-term risk of an adverse health effect. Rather, such information indicates the need for careful monitoring and the assessment of longer-term data trends.

WTC screening levels
antimony - 2 ug/m3**
arsenic - 0.3 ug/m3
beryllium - 0.02 ug/m3**
cadmium - 0.2 ug/m3
cobalt - 1.0 ug/m3**
nickel - 2 ug/m3
manganese - 0.5 ug/m3
selenium - 200 ug/m3**

** The unit, ug/m3, is the mass of metal in micrograms (a millionth of a gram) in a cubic meter of air.