Free software tends to have fewer bugs and faster development than commercial software.
In addition, it is usually very high in quality, because the people who develop it are the people who use it. We are developing our exposure modeling framework inside the R system. R is Free Software and is very similar to S and Splus . Ultimately we will release an exposure modeling R package that contains data and functions for the study of human exposure to toxic pollutants. The package will be freely available and licensed under the GNU General Public License.