Unit 2: Positive externalities: Fluoride in water

The BBC reports that the government wants areas with high levels of tooth decay to consider fluoridation in water.

< http://www.bbc.co.uk/mediaselector/check/player/nol/newsid_7220000/newsid_7227600?redirect=7227615.stm&news=1&nbwm=1&bbram=1&nbram=1&bbwm=1&asb=1 >

a) Define what is meant by a positive extenality
b) Using the definition, explain the likely positive externalities that will arise from fluoride in water