Sodium fluoride will be added to Cambridge's water late this month, Dr. Edward O'Rourke, Commissioner of Public Health, announced yesterday. At the same time a University expert warned the chemical might prove harmful to certain users, but claimed the risk of danger was small.

Smal quantities of sodium fluoride are to be mixed with drinking water. This process is calculated to decrease sharply the dental decay rate of children, and to a lesser extent that of adults.

Dr. James H. Shaw, Chairman of the Harvard School Committee on Fluoride, said that its addition can not be detected by smell, taste, or color, and will not be harmful to the average healthy person. There is, however, a "calculated risk" that the fluoridation could be detrimental to sufferers from diabetes and certain kidney diseases, but the risk is very small compared to the good that can be done.

Fluoride for Cambridge water has been under discussion since 1948 when the late Dr. Autino Fioro, then on the staff of the School of Public Health, suggested it after observing the high rate of tooth decay of Cambridge children.

Cambridge is able to undertake the project because it is not part of the metropolitan Boston water supply system.