A lot of people—and a number of water filter companies—would have you believe that TDS (total dissolved solids) meter readings are the best measure of water’s overall suitability for drinking, which is totally false.
TDS meters are generally easy to use, and while they can tell you certain things about your water, a TDS meter cannot tell you if your water is safe to drink based solely on the levels of total dissolved solids. Contrary to some of the advertising used by reverse osmosis (RO) system and water pitcher brands you may have seen or heard about, a TDS meter only tells a small part of the story when it comes to evaluating the suitability of water for drinking.
What Can a TDS Meter Tell You? How Does It Work?
Dissolved solids in water such as salts, minerals and heavy metals contain charged particles: negatively charged (anions), and positively charged (cations). Most TDS meters simply use electrodes to measure the resulting electrical conductivity of the water, which the meter converts to a TDS value.
Water with higher levels of dissolved solids will have greater electrical conductivity (and thus higher TDS meter readings), and water with lower levels of dissolved solids will have a lower electrical conductivity (lower TDS meter readings). TDS meter readings are usually expressed in parts per million (ppm), which is also the same as mg/L.
TDS meters can be useful for confirming the removal of total dissolved solids if your home or business utilizes a water distillation or reverse osmosis system. TDS readings can also help you evaluate the hardness of your water and if a water softener system might be needed, or if scale buildup/deposits in your pipes might be related to excess minerals present in your water.
What are Examples of Total Dissolved Solids?
Dissolved solids include salts, minerals (many of which can be beneficial to your health) and heavy metals (which may be bad for you). Some of the most common dissolved solids include aluminum, calcium, magnesium, chloride, potassium, sulfate, nitrate, silica, iron, lead, potassium, sodium and zinc.
What are Sources of Total Dissolved Solids, Both Good and Bad?
- Urban and agricultural runoff
- Industrial waste water
- Saltwater intrusion into freshwater sources used by municipal water supplies
- Water pipes in your home
- Road de-icing salts
- Natural sources– aquifers, springs (which are often rich in beneficial mineral content)
Are There Acceptable Levels of Total Dissolved Solids in Drinking Water?
Total dissolved solids are considered by the Environmental Protection Agency (EPA) to be a “nuisance chemical”, since they can affect taste, color and odor of drinking water. Therefore, the EPA has set non-enforceable secondary maximum contaminant levels (also known as SMCL) for total dissolved solids and other contaminants that are not considered to present a risk to human health at the SMCL.
The secondary maximum contaminant level for total dissolved solids is 500 mg/L. Water containing levels of total dissolved solids above the EPA’s recommended secondary maximum contaminant level of 500 mg/L would likely not be very appealing to drink. It could have a salty taste or have a less-than-clear color. Other effects of high levels of total dissolved solids in drinking water can include water hardness, scale formation and staining.
Before You Buy Into the Hype of “0″ TDS Claims, Consider the Following:
- While a “0” TDS reading would indeed indicate no dissolved solids present, a “0” TDS reading also indicates that no beneficial minerals (that your body needs) remain in the drinking water. Many consumers prefer filter systems that allow the water to retain beneficial minerals that can help support the body’s ability to stay hydrated. Plus, “0” TDS water—essentially the same as distilled water you’d use in a clothes iron, CPAP machine or aquarium—will have very little taste, if any, or a “flat” flavor due to the absence of minerals or impurities. On the other hand, some people are willing to pay top dollar to buy mineral water, which is touted to be healthier and taste great. But due to minerals in the water that contribute to the taste, TDS readings of the top-selling brands of mineral water can often range from 200 up to 2000+ (doing a web search for ‘popular mineral water TDS levels’ can be quite eye-opening). So, it begs the question: is “0” TDS (or low readings) really the best way to determine water quality?
- Depending on the drinking water source (especially well water or other questionable tap water sources such as at a campground or state park), filtering that water through some systems (such as a water filter pitcher) could still give you a “0” TDS reading, yet the water may still be contaminated with typical contaminants such as giardia, cryptosporidium, bacteria (e. coli, etc.), toxic chemicals, or any number of other water contaminant threats that are undetectable by TDS meters.
- In a more extreme scenario (just to make a point), even though the juices from a package of raw chicken that’s been sitting out on the counter for hours could hypothetically register “0” TDS, you would never drink it due to the likely presence of bacteria.The same reasoning applies to drinking water: unless it’s tested for specific threats or filtered by a powerful and proven water filter system, you really don’t know what you’re drinking.
What Can a TDS Meter Not Measure?
TDS meters can only measure the levels of electrical conductivity in drinking water caused by dissolved solids. Some models can measure water temperature, pH level and salinity as well. However, TDS meters cannot tell you which dissolved solids (good or bad) or contaminants might be present. For a specific breakdown of dissolved solids or other contaminants in your water, a more thorough water test is required.
And because a TDS meter only detects charged ions, a TDS meter cannot measure uncharged contaminants. Uncharged contaminants include a number of contaminants that affect the safety of drinking water.
Some of the more concerning uncharged contaminants that can be found in drinking water and are not detected by a TDS meter include, but are not limited to:
- Perfluorinated “forever chemicals” such as PFAS
- Potentially lethal biological microorganisms/pathogens such as viruses, bacteria and parasites such as cryptosporidium and giardia, etc.
- Pesticides and other toxic chemicals
- Hexavalent chromium
- Petroleum products
- Other various types of contaminants that could be present in your drinking water
The Bottom Line
Again, TDS meters can only measure “nuisance chemicals” affecting taste, odor and appearance, but a TDS meter won’t tell you if (or which) much more potentially harmful contaminants could be present in your drinking water that could present both immediate health risks (biological contaminants or toxic chemicals) or long-term health risks (such as PFAS, lead, etc.). Relying on a TDS meter to determine if drinking water is truly suitable for your family to consume is a risky and foolish gamble at best.
Moreover, it should be noted that a lot of water filter companies—some perhaps dishonestly—continue to run ads showing a TDS meter displaying a low reading such as “0” or “5” on the screen, and implying or even directly stating that due to low TDS readings, the water filtered by their system is the best or safest for you to drink.
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