What Makes Drinking Water Safe? Key Quality Standards to Follow

Access to safe drinking water is one of the most fundamental public health requirements, yet waterborne diseases continue to affect millions of people worldwide every year. The safety of drinking water is not judged by appearance alone – it depends on a set of measurable chemical, physical, and biological parameters that must consistently fall within acceptable limits. Organizations like the World Health Organization (WHO) and India’s Bureau of Indian Standards (BIS) have developed detailed guidelines to define exactly what “safe” means when it comes to the water we drink.

Why drinking water quality standards exist 🔗

Water is a universal solvent – it picks up a wide range of substances as it moves through soil, rock, and distribution pipes. Some of these substances are harmless or even beneficial in small quantities, but others are dangerous at any detectable level. The challenge for water managers and public health authorities is to monitor these substances continuously and ensure that water supplied to communities stays within safe limits.

WHO’s Guidelines for Drinking-water Quality (GDWQ), now in its fourth edition, has built on over six decades of research to establish health-based targets that countries can use to develop their own national standards. In India, the BIS standard IS:10500-2012 serves this purpose, aligning closely with WHO recommendations while also accounting for local conditions. Countries without a strong legislative framework often look to WHO guidelines as the default reference for setting safe limits.

These standards cover a broad range of parameters – physical properties like turbidity and color, chemical parameters like pH, hardness, and total dissolved solids (TDS), the presence of toxic metals and organic chemicals, and biological contamination from bacteria and viruses.

Physical parameters: what you can see and sense 🔗

Turbidity 🔗

Turbidity is a measure of how much suspended matter is present in water, causing it to appear cloudy or hazy. It is measured in Nephelometric Turbidity Units (NTU). According to the U.S. Geological Survey (USGS), turbidity is not a direct health risk in itself, but it is an important warning indicator. High turbidity means there are particles in the water that can shelter pathogens from disinfectants like chlorine, allowing harmful microbes to survive water treatment and reach consumers.

Turbid water is also harder to treat effectively. When turbidity rises, chemical disinfectants and UV light become less efficient because particulate matter physically blocks their action. This can result in dangerous bacteria, protozoa like Giardia and Cryptosporidium, and viruses remaining in treated water. The WHO and BIS both set an acceptable turbidity limit of 1 NTU for treated drinking water, with a permissible relaxation to 5 NTU only where no better alternative source is available. The U.S. EPA requires treated water turbidity to remain at or below 1 NTU, with 95% of monthly samples meeting a tighter target of 0.3 NTU.

Color and odor 🔗

Safe drinking water should be colorless and odorless. The presence of color in water typically indicates organic matter, iron, manganese, or industrial pollutants. BIS specifies a maximum acceptable color value of 5 Hazen units, with a permissible limit of 15 Hazen units in the absence of an alternative. Similarly, water should have no objectionable odor – any persistent smell of sulfur, chlorine, or earthiness can indicate contamination or treatment failure.

Chemical parameters: the hidden threats 🔗

pH level 🔗

The pH of water indicates its acidity or alkalinity on a scale of 0 to 14, with 7 being neutral. Numbers below 7 indicate acidity, and numbers above 7 indicate alkalinity. For drinking water, both WHO and BIS recommend a pH range of 6.5 to 8.5. Water outside this range can cause problems: highly acidic water corrodes metal pipes, potentially leaching lead and copper into the supply, while highly alkaline water tastes bitter and can reduce the effectiveness of disinfection processes.

Maintaining the correct pH is also critical during water treatment itself. Coagulation, chlorination, and other treatment steps work most effectively within a specific pH window, so utilities must constantly monitor and adjust pH to ensure that treatment remains effective and that the final water is safe to drink.

Total dissolved solids (TDS) 🔗

Total dissolved solids (TDS) refers to the combined concentration of all dissolved inorganic and organic substances in water – including salts, minerals, and metals. TDS is made up of inorganic salts as well as a small amount of organic matter, including calcium, magnesium, potassium, chlorides, sulfates, and nitrates. These can originate from natural rock weathering, agricultural runoff, or industrial discharge.

WHO recommends a TDS level below 500 mg/L for drinking water, and BIS IS:10500-2012 sets an acceptable limit of 500 mg/L with a permissible upper limit of 2000 mg/L where no other source exists. High TDS concentrations can cause adverse taste effects and may also cause deterioration of domestic plumbing and appliances. Although TDS itself is not classified as a direct health hazard, very high concentrations often signal the presence of other contaminants that may be harmful.

Hardness 🔗

Hardness is caused by elevated concentrations of calcium and magnesium dissolved in water. While moderately hard water is not harmful to health – and in fact contributes some essential minerals – very hard water causes scaling in pipes and water heaters, reduces the lathering of soaps, and can affect the taste of water. BIS sets an acceptable hardness limit of 200 mg/L and a permissible limit of 600 mg/L. WHO does not set a specific health-based limit for hardness but recommends monitoring it as part of overall water quality management.

Nitrates 🔗

Nitrates enter drinking water primarily through agricultural runoff, where fertilizers and animal waste leach into groundwater. Nitrate levels should ideally not exceed 10 mg/L (as nitrogen), since elevated concentrations can cause methemoglobinemia – commonly called “blue baby syndrome” – in infants, a condition where the blood’s oxygen-carrying capacity is severely reduced. WHO and BIS both set a maximum acceptable nitrate limit of 45 mg/L, equivalent to 10 mg/L as nitrogen. This is a particularly important parameter in agricultural regions where groundwater is the primary drinking source.

Toxic chemical contaminants 🔗

Heavy metals 🔗

Heavy metals are among the most serious chemical threats to drinking water safety. Heavy metals can leach into drinking water from household plumbing and service lines, mining operations, petroleum refineries, and natural mineral deposits. The key metals monitored in drinking water include arsenic, lead, cadmium, chromium, mercury, and iron.

Arsenic is particularly dangerous. Its inorganic forms are highly carcinogenic and have been linked to cancers of the lungs, liver, bladder, and skin. WHO sets a maximum arsenic limit of 10 µg/L in drinking water. Heavy metal contamination in drinking water is associated with cardiovascular disorders, neuronal damage, renal injuries, and increased cancer risk. Lead, which often enters water through corroded old pipes rather than from pollution sources, is harmful even at very low levels and has no safe exposure threshold. The WHO guideline value for lead in drinking water is 10 µg/L.

The BIS IS:10500-2012 standard sets acceptable limits for a range of heavy metals, with strict “no relaxation” clauses for the most toxic ones, meaning water exceeding these limits is simply unfit for consumption regardless of circumstances.

Fluoride 🔗

Fluoride occupies a unique position in water quality standards – it is beneficial in controlled amounts but harmful in excess. At concentrations between 0.5 and 1.5 mg/L, fluoride helps prevent tooth decay. However, excessive fluoride consumption can cause skeletal fluorosis, a condition characterized by pain and tenderness in bones and joints. WHO and BIS both set a maximum acceptable fluoride limit of 1 mg/L, with a permissible limit of 1.5 mg/L. In parts of India, groundwater fluoride levels naturally exceed these limits, posing a chronic health risk to communities dependent on such sources.

Pesticides and organic chemicals 🔗

Agricultural runoff, industrial discharge, and improper waste disposal introduce a range of organic chemicals and pesticide residues into water sources. People who consume high levels of organic chemicals may suffer damage to their kidneys, liver, circulatory system, nervous system, and reproductive system. The WHO’s chemical hazards guidelines cover a wide range of pesticides and organic pollutants, with each substance assigned a guideline value based on its specific toxicity profile. BIS IS:10500-2012 specifies that total pesticide residues in drinking water should not exceed 0.0001 mg/L for individual pesticides and 0.0005 mg/L in total.

Biological parameters: the most immediate health threat 🔗

Biological contamination from bacteria, viruses, and protozoa represents the most immediate risk to drinking water safety. Coliform bacteria, particularly Escherichia coli (E. coli), serve as the key indicator organisms for fecal contamination. Both WHO and BIS specify that no coliform bacteria should be detectable in any 100 mL sample of treated drinking water – this is an absolute standard with no relaxation. The presence of E. coli indicates the possible presence of other fecal pathogens including cholera-causing Vibrio cholerae, typhoid-causing Salmonella typhi, and protozoa such as Giardia and Cryptosporidium, which cause severe gastrointestinal illnesses.

Research has consistently found positive associations between elevated drinking water turbidity and increased cases of acute gastrointestinal illness, underlining how closely physical, chemical, and biological parameters are linked in determining overall water safety.

The WHO and BIS framework: how standards are set and used 🔗

The WHO Guidelines for Drinking-water Quality promote protection of public health through health-based targets, water safety plans covering the entire supply chain from catchment to consumer, and independent surveillance. The approach emphasizes prevention rather than just end-point testing – identifying risks at every stage of the water supply system and managing them proactively.

India’s BIS IS:10500-2012 standard draws directly from WHO guidelines while also accounting for domestic conditions. It provides two tiers of limits for most parameters: an acceptable limit that should always be achieved, and a more relaxed permissible limit that applies only when no alternative source is available. For parameters like arsenic, lead, E. coli, and pesticides, there is no permissible relaxation – these are hard limits.

In practice, water utilities use routine sampling and laboratory testing to monitor all these parameters at multiple points – from source water to treatment stages to distribution. Where limits are exceeded, treatment must be adjusted or the supply must be suspended until the water is brought back into compliance. Technologies like reverse osmosis (RO), UV disinfection, coagulation-flocculation, chlorination, and activated carbon filtration are used to address different types of contamination.

Why monitoring alone is not enough 🔗

Standards and testing are essential, but they must be supported by strong infrastructure and governance. The effect of chemical contaminations in drinking water tends to be chronic rather than acute – it can act as a silent killer over years of exposure. Communities relying on aging pipelines, unmonitored bore wells, or inadequately treated surface water face ongoing risks that routine testing schedules may not always capture in time.

Research also highlights that WHO standards are set for individual contaminants, but synergic effects of multiple chemicals like cadmium, fluoride, and water hardness can cause severe kidney damage even when each individual parameter is within the recommended limit. This means that comprehensive water safety requires not just meeting individual parameter thresholds, but understanding how multiple contaminants interact – a challenge that demands continued research and regulatory refinement.

What do you think? If most waterborne health risks from chemical contamination are chronic and slow to manifest, how should communities prioritize water quality monitoring – through regular household testing, centralized utility surveillance, or both? And given that standards like BIS and WHO set limits for individual contaminants separately, should future guidelines also account for the combined effects of multiple chemicals present simultaneously in drinking water?

References

1. https://www.who.int/teams/environment-climate-change-and-health/water-sanitation-and-health/water-safety-and-quality/drinking-water-quality-guidelines
2. https://cpcb.nic.in/wqstandards/
3. https://www.who.int/publications/i/item/9789240045064
4. https://en.wikipedia.org/wiki/Drinking_water_quality_standards
5. https://www.usgs.gov/special-topics/water-science-school/science/turbidity-and-water
6. https://dropconnect.com/turbidity-in-drinking-water/
7. https://www.epa.gov/privatewells/potential-well-water-contaminants-and-their-impacts
8. https://deq.nd.gov/chemistry/reports.aspx
9. https://www.safewater.org/fact-sheets-1/2017/1/23/tds-and-ph
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC4427717/
11. https://pubmed.ncbi.nlm.nih.gov/28643849/
12. https://www.who.int/teams/environment-climate-change-and-health/water-sanitation-and-health/chemical-hazards-in-drinking-water
13. https://pmc.ncbi.nlm.nih.gov/articles/PMC2174477/
14. https://www.nature.com/articles/srep42516