Section 8C. The department in taking cognizance of the dental health of the people in the commonwealth shall recommend such methods as in its opinion are advisable to reduce or limit the prevalence of dental caries and other dental diseases and defects. If the commissioner determines that the fluoride content of the public water supply for domestic use in any city, town or district is not at optimum level for sound dental health, he shall so notify the local board of health of his findings. Such board of health, after making such inquiry and other use of the consulting services of the department or elsewhere as it chooses, shall, if it considers doing so in the best interest of the inhabitants of the city, town or district within its jurisdiction, order the upward adjustment of the fluoride content of the water supply available for domestic use in that city, town or district. No such order shall be effective until ninety days after it has been published in a newspaper having a general circulation in such city or town, or until favorable vote has been taken in accordance with the provisions of this section, whichever occurs later.

The provisions of this section shall not apply if two or more cities or towns are supplied water from the same source, if such supply to each city or town cannot be treated independently and if the majority of the boards of health representing such cities and towns have voted not to accept such recommendation; provided, however, that any such city or town desiring to adjust upward the fluoride content of the water consumed within its own jurisdiction may comply with the order by the installation of proper equipment that will comply therewith if it does not interfere with the water supply of said other cities or towns.

In any city, town or district where the board of health has ordered the upward adjustment of the fluoride content of the water supply under the provisions of this section, upon petition of ten per cent of the registered voters of said city, town or district, filed in the office of the city or town clerk, as the case may be, within ninety days of the publication of such order, the following question shall be placed upon the official ballot to be used at the next regular city election or for the election of town officers at the next annual town meeting or at a biennial state election, whichever occurs first, but not earlier than sixty days following the date of filing the petition with the city or town clerk:—''Shall the public water supply for domestic use in (this city) (this town) be fluoridated?'', or in such district the following question shall be placed before the next annual meeting of the inhabitants of the district:—''Shall the public water supply for domestic use in this district be fluoridated?'' If the majority of votes in answer to said question is in the negative, the water supply of such city, town or district shall not be fluoridated, and the fluoridation of such water supply shall not be ordered again by the board of health for a period of at least two years from the date of such vote.