Environmental factors causing ALS?

My sister has worked at the same place for more than 30 years. Some years back there was a toxic mold problem and the business had to close for hazmat crews to come in and make it safe. Awhile later, a co-worker in the warehouse part of the business was diagnosed with ALS. He is now deceased. Prior to the clean-up, my sister and her co-workers were ill for a couple of years with upper respiratory problems, sore throats, etc. Then the mold was discovered and subsequently eliminated (hopefully). Later, the worker was diagnosed with ALS. This month - January - my sister was diagnosed with ALS. Two people in a rather small business with ALS. There are maybe two dozen workers. So if 6,000 people each year in the US are diagnosed with ALS - this means there are two per 100,000 people in our population. At my sister's company there were two out of maybe 24 give or take. Is this a grim coincidence or could it be something more? Anyone?