| Arsenic | EPA MCL: 0.010 mg/L | Milligrams per liter | Concentrations should be at or below the federal maximum contaminant level. | Long-term exposure may increase the risk of cancer and other health effects. | Validated arsenic reduction; reverse osmosis or an appropriately certified adsorption medium may be suitable. |
| Lead | Action level: 0.015 mg/L at the 90th percentile | Milligrams per liter; usually assessed at residential taps | Lead control focuses on corrosion prevention and tap-level monitoring rather than a conventional MCL. | Particularly harmful to infants, children, and fetuses; there is no known safe level of exposure. | Use a point-of-use filter specifically tested for lead reduction and address plumbing or corrosion sources. |
| Nitrate | EPA MCL: 10 mg/L as nitrogen | Milligrams per liter as nitrogen | Water at or below the MCL meets the federal drinking-water limit for nitrate. | High levels can cause methemoglobinemia, especially in young infants. | Reverse osmosis or ion exchange designed and tested for nitrate reduction; boiling does not remove nitrate. |
| Nitrite | EPA MCL: 1 mg/L as nitrogen | Milligrams per liter as nitrogen | Water at or below the MCL meets the federal drinking-water limit for nitrite. | Can interfere with oxygen transport in the blood, with infants at greater risk. | Use a treatment process specifically validated for nitrite; confirm performance through laboratory testing. |
| Fluoride | EPA MCL: 4.0 mg/L; secondary standard: 2.0 mg/L | Milligrams per liter | The MCL protects against skeletal fluorosis; the secondary standard addresses cosmetic dental effects. | Excessive long-term exposure can affect bones and teeth. | Reverse osmosis or activated alumina may reduce fluoride when the system is properly designed and maintained. |
| Total dissolved solids (TDS) | EPA secondary standard: 500 mg/L | Milligrams per liter; commonly estimated by conductivity meters | TDS is an aesthetic and operational indicator, not a complete measure of safety. A low TDS reading does not prove that water is pathogen-free. | High TDS may cause salty taste, scaling, staining, and reduced suitability for some appliances. | Reverse osmosis typically provides substantial TDS reduction; verify contaminant-specific performance separately. |
| Total hardness as calcium carbonate | No federal EPA MCL; common classification: soft <60 mgl, moderately hard 60–120 121–180very>180 mg/L | Milligrams per liter as CaCO3 | Hardness is mainly a scaling, taste, and cleaning concern rather than a federally regulated health limit. | Hard water can cause scale, reduce soap efficiency, and shorten the life of heating elements and membranes. | Water softening or antiscalant control for whole-home protection; reverse osmosis for drinking-water polishing. |
| E. coli | Should be absent in a 100 mL drinking-water sample | Presence or absence in 100 milliliters | Detection indicates possible fecal contamination and requires prompt investigation and corrective action. | May indicate disease-causing microorganisms, including bacteria, viruses, or parasites. | Validated microbiological barrier such as ultrafiltration, ultraviolet disinfection, or properly maintained reverse osmosis; disinfection alone does not remove chemicals. |
| Total coliform bacteria | Compliance is based on the applicable microbial treatment technique and distribution-system requirements | Usually reported as presence/absence or colony-forming units | Total coliforms are indicator organisms; a positive result can signal treatment, plumbing, or storage problems but does not by itself prove fecal contamination. | May indicate conditions that allow microbial contamination to enter or grow in the water system. | Choose a system with a validated microbiological reduction claim and maintain filters, storage tanks, and sanitary connections. |
| Turbidity | Operational target: as low as practical; public systems commonly use 0.3 NTU as a treatment-performance benchmark | Nephelometric turbidity units | Cloudiness can reduce disinfection effectiveness and may indicate suspended particles or treatment problems. | Particles can shield microorganisms and increase fouling of downstream treatment stages. | Sediment filtration or ultrafiltration before fine purification and disinfection stages. |
| pH | EPA secondary range: 6.5–8.5 | pH units | pH affects taste, corrosion, scaling, and the performance of some treatment processes. | Very low or high pH can contribute to corrosion, metal leaching, or scale formation. | Consider prefiltration, corrosion control, remineralization, or pH adjustment based on laboratory results. |
| Residual disinfectant | Follow the local utility or regulatory operating target | Usually measured as mg/L of free chlorine or chloramine | A disinfectant residual helps protect water in distribution pipes but can affect taste and odor. | Insufficient residual may permit microbial regrowth; excessive levels may create taste and odor concerns. | Activated carbon can reduce chlorine taste and odor; do not remove disinfectant without maintaining hygienic storage and plumbing. |