Question: What procedure is best to determine background airborne silica levels?

Hi Matthew, We met briefly years ago when you visited with Mike O'Connor in Buffalo County Wisconsin. Since then I've hopefully learned a bit more and helped out with analysis of a University of Iowa monitoring project in Trempleau county. I'm sure you are familiar with the Richard's study of silica at the EOG facility in Chippewa Falls and I would hope you are familiar with the U of Iowa study which included silica sampling around a few homes. Neither of these showed silica concentrations anywhere near the current suggested health guidelines. Yes, Richards was employed by the industry and one can see where his methods could have been improved, but there is no evidence of data manipulation. I bring these up in relation to your second question about "statistical significance". You haven't given any explanation of what this might mean to you. Often it's a statement of whether whatever is being measured is different from zero. To make an assessment of this and to suggest a sample size one needs an estimate of variability. If you do not use these past studies to give a reasonable estimate you have nothing to begin with and need to go out and monitor a lot! You need to do this anyway because outdoor ambient monitoring needs to consider all kinds of weather conditions. This is not a controlled environment. In addition if you want to try and assess what might be coming from a facility you need to be sure your baseline is NOT coming from the facility. Then the difference might be relevant to your question about the facility. I write "might" because there is still the work of trying to attribute the difference to the facility. The complexities of this type of analysis is something many people do not understand. These studies can not be done quickly. I suspect you know all this. In addition there is the question of whether work at a facility is consistent enough that a 24 hour average concentration will answer the community's questions. I've witnessed and photographed a mine blast and the corresponding dust cloud. One could see the dust cloud for less than 15 seconds before it had dispersed enough to no longer be identifiable. Concentrations of silica could have been very high within the cloud produced but the cloud was no where near where I was standing and even if it was, very high concentration for 15 seconds out of 24 hours would not have a discernible signal unless the concentration was truly enormous. 24 hour averaging is problematic by itself and in my opinion should always be used in association with pm monitoring of at most every minute. It is the case that concentrations of silica are usually so low that a monitoring period has to be of sufficient length that enough accumulates on a filter to be measurable. This makes getting answers to questions much more difficult but also pushes one to be creative. jeff falk