Random Thought: Post surgical infections,

The New York Times published an interesting article (http://www.nytimes.com/2010/01/07/health/research/07infection.html?ref=science )describing medical studies stating that many of the infections that people get in hospitals are caused by bacteria that they bring in. In these cases, potential solutions are knocking down these bacteria before surgery and this is being done. However, it appeared that more could be done and there was the striking point made that chlorhexidine-alcohol, worked much better for surgical prep than povidone-iodine but the povidone-iodine was used because the cost per patient was $3.50 as opposed to $12 for chlorhexidine - alcohol.

It would be stunning if true: saving $8.50 but raising the risk of infection! It is hard to find an operation that needs a serious surgical prep for which the surgeon's bill would not be more than $5000!

Other than sanity and cost analysis, what is the connection with chemical engineering? A simple model for bacteria growth is that each bacterium splits into two daughters every growth time period. This mechanism leads to exponential growth in the number of bacteria. The time period depends on a lot of things, but 1/2 hour could be used as a nominal value. So why would a better disinfectant be useful? For this growth period, in 24 hours the original number of bacteria will be multiplied by about 163,000. Thus if the original disinfection leaves any significant amount of bacteria, fairly large concentrations could reappear in a day (as a point of comparison: 35 enterococci per 100 ml correlates to an expected rate of 19 diarrheal diseases per 1,000 swimmers -- http://ga.water.usgs.gov/projects/chatm/importance.html). So the need to kill almost all of the bacteria present before surgery is really important!