Fluoride testing tap water, retail-purchased bottled spring water, and distilled water

Looks like 0.5 mg/L (equivalent to 0.5ppm) of fluoride in our tap water.

According to the Use fluoride page at toothclub.gov.hk, the fluoride concentration of our tap water should be 0.5 mg/L (equivalent to 0.5ppm):

Domestic water in Hong Kong has been fluoridated since 1961. This has led to a great decline in the incidence of tooth decay. Today in Hong Kong, the fluoride concentration in water is 0.5 ppm (parts per million). By drinking fluoridated water, the intake of fluoride is adequate to prevent tooth decay.

It used to be much higher. A paper published in the journal Advances in Dental Research in 1989 (Changes in dental fluorosis following an adjustment to the fluoride concentration of Hong Kong’s water supplies) found a reduction in dental fluorosis in children whose permanent teeth formed after the Hong Kong authorities reduced the fluoride concentration target for local drinking water from 1mg/L to 0.7mg/L in 1978. At some point after 1989, that was cut again to 0.5mg/L.

U.S. Air Force Brigadier General Jack D. Ripper bringing certain facts to the attention of his executive officer, Group Captain Lionel Mandrake (on loan from the U.K. RAF). — Mandrake, have you ever seen a commie drink a glass of water?

By the way, and on a completely unrelated note, if you haven’t watched the film Dr. Strangelove, you’re missing out! One of my favorite scenes in the movie (Dr. Strangelove (4/8) Movie CLIP – Water and Commies (1964) HD) is the one in which fictional U.S. Air Force Brigadier General Jack D. Ripper brings certain facts to the attention of his executive officer, fictional Group Captain Lionel Mandrake (on loan from the U.K. RAF). Anyway, I digress.

The fluoride test kit I purchased on Taobao (after looking on Amazon and not seeing anything with the same granularity at lower levels) seemed to measure the concentration in our tap water accurately. I also tested some bottled spring water, from two brands that we often consume, as well as some of the distilled water we use for our espresso machine and for cooking:

Left to right: bottled spring water #1, bottled spring water #2, and distilled water.

Lighting for photography and color-matching and color-grading photos and video is tricky business, at least for a non-professional-photographer like yours truly. The test solution in the center (second brand of bottled spring water) looks more red or more vibrantly red than the ones on either side, but some of the circular color chips on the colorimetric chart card also look more vibrant than they do in the other pictures that I smooshed together to make that collage. In person, when we eyeballed them, S. and I found two of the three to either match the color chip corresponding to 0 or to be somewhere between 0 and 0.1 mg/L. The first brand of spring water definitely seemed to have a wee twinge of fluoride in it, however.

Some photos (cropped screencaps of Google Translate in action) of the kit and some details in the caption for each: