
EPA Point Count 400 and 1000
When a sample is tested positive for the presence of asbestos, and the percentage is 10% or lower, a point counting method is recommended for that sample. If the results derived from point count differs from the original results, it overrides it.
Point counting is an analytical technique used to determine the quantity of a component in a specific type of materials. Originally point counting was used for determining the percentage of various minerals in a thinsection of rock. Point Counting can also be applied to the quantification of asbestos in various building materials. EPA requires counting a minimum 400 total nonempty points. The State of California requires counting 1000 total nonempty points.
Most of the time, a point counting is requested by the client to see the whether the sample contains less than one percent of asbestos, below which a material is legally considered nonasbestos containing. It makes little
difference if a material is determined to be 10 percent or 11 percent asbestos, however, it
is significant if the level is determined to be less than one percent or exactly one percent.
In point counting, a graticule is
placed in the ocular (eye piece) of the microscope. The graticule contains a crosshair.
The place at which the crosshairs meet is known as the “point? When the slide containing
the sample preparation is moved into the field of view of the microscope the “point?may
fall on a particle. If the particle is identified as an asbestos fiber the point is counted as a
asbestos point. If the particle is identified as a nonasbestos particle then the point is
counted as a non asbestos point. If the point falls on an empty area of the slide nothing is
counted and the slide is moved to a new field of view. The process is repeated until 400
to 1000 asbestos and non asbestos points are counted. The total number of asbestos
points is found and is divided by the number of non asbestos points and asbestos points
counted. The result is multiplied by 100. This gives the percent of asbestos in the
sample.
There are limitations to this method. The size of the asbestos fibers themselves play an important role in quantification. If the size of the asbestos fiber is too small, and doesn’t fall on the crosshair, it will be overlooked. Second,
Point counting is statistical. Obviously one can not look at the entire sample to determine
the concentration of asbestos. Therefore, the field technician must take a
representative sample of the material and send that to the lab. Once the sample is in
the laboratory the analyst must prepare slides that are representative of the sample
sent in by the client
As mentioned before, according to
regulations if the sample is 1 percent or greater than it is considered to be an asbestos
containing material.
When performing a 400 point count analysis of an asbestos sample that contains 1%
asbestos we should count 4 asbestos particles in 400 total particles because there is a
probability in 400 trials that 4 particles should be asbestos and 396 should not. The 95%
confidence level of the sample containing 1% asbestos would be 4 asbestos particles +/
3.9 asbestos particles. Therefore we are 95% certain that our analysis of a sample that
actually contains 1% asbestos will give a result between 0 to 2% asbestos. For a 1000
point count of a 1% asbestos sample we should count 10 asbestos particles. The 95%
confidence level of this will be smaller because the number of points counted is higher.
Therefore, we would expect to count 10 +/ 6.17 asbestos particles for a range of 1.6% to
0.4% asbestos. In either case we can’t be exactly certain that the sample contains 1%
asbestos but that it falls into a range between the 95% upper confidence limit and the
95% lower confidence limit. When interpreting the results of analysis one must keep this
fact in mind.