Naturally Occurring Asbestos

Activity-based sampling – Activity-based sampling of the air in the breathing zone of an individual while that individual participates in typical work or recreational activities.  It has been used for decades by industrial hygienists to measure personal exposures in workplace environments.  It is more representative of actual individual exposures than fixed, stationary monitors and soil sampling, and is being used by EPA to sample exposures at asbestos sites across the country.

Ambient Air – Ambient air is surrounding air that is not immediately affected by a disturbance or activity.

Amphiboles – One of two mineral families which contain asbestos minerals.  Amphibole asbestos tends to form in needle-like shapes and includes tremolite, actinolite, winchite, richterite, anthophyllite, crocidolite, and amosite asbestos.

Chrysotile – Asbestos from the serpentine family of minerals.  Chrysotile asbestos is flexible and historically accounts for about 95% of the asbestos used commercially in the United States.

Excess Lifetime Cancer Risk – An estimate of the probability that a person may develop cancer in excess of background rates sometime in his or her lifetime following exposure to a particular contaminant. 

Mesothelioma – A rare cancer which may affect the lining of the lungs (pleura) or the abdominal contents (peritoneum). Most mesotheliomas are caused by exposure to asbestos.  Most cases of mesothelioma are diagnosed 30 years or more after the first exposure to asbestos.

PCME or Phase Contrast Microscopy Equivalent – Phase Contrast Microscopy (PCM) is the analytical method for asbestos used in occupational environments.  Current health standards for asbestos are based on studies which document the adverse health effects from asbestos exposure in workers.  Since the worker exposures used PCM, the health standards are based on the fibers which are counted in PCM.  Today, EPA uses an analytical technique, Transmission Electron Microscopy or TEM, which can see much smaller and thinner fibers (20,000 magnification vs. 400 magnification).  In order to apply the current health standards to fibers which were counted using a TEM analysis, the EPA used only those fibers which would have been seen in the PCM method, and this equivalent count of fibers is called the Phase Contrast Microscopy equivalent or PCME.  For this risk assessment, only those fibers which are considered to be of PCME dimensions, greater than 5 microns in length, with at least a 3:1 length to width ratio, and a diameter between 0.25 microns and 3 microns inclusive, were used.  To give some idea of the size of a PCME fiber, the average width of a human hair is 80 microns.

95% UCL or 95% Upper Confidence Limit of the Mean – A statistical calculation of the mean concentration so that the actual mean will be less than this value 95% of the time.