The World Health Organization publishes a set of Guidelines for Drinking-water Quality (GDWQ) that serve as the international benchmark for safe drinking water. These guidelines set concentration limits for chemicals, microbes, and radioactive substances, and they form the basis for national regulations in countries around the world. The current version is the fourth edition, last updated in March 2022 with corrections added in May 2023.
The WHO guidelines are not legally binding on any country. Instead, they are designed as a starting point that each nation adapts into its own enforceable standards based on local conditions, water sources, and resources. The U.S. Environmental Protection Agency, the European Union, and individual governments in low- and middle-income countries all draw on these guidelines when writing their own rules.
What the Guidelines Cover
The GDWQ address four broad categories of contamination: microbiological (bacteria, viruses, parasites), chemical (metals, pesticides, industrial pollutants), radiological (radioactive particles), and aesthetic qualities (taste, odor, appearance). For each contaminant, WHO establishes a guideline value, which is the maximum concentration considered safe for lifelong consumption. These values are calculated to protect the most vulnerable groups, including infants, pregnant women, and people with compromised immune systems.
Beyond setting numbers, the guidelines promote a prevention-first approach called the Water Safety Plan. Rather than relying solely on testing finished water, this framework asks water suppliers to assess risks across the entire supply chain, from the original water source through treatment and distribution all the way to the tap. A Water Safety Plan has three core components: a system assessment of the full supply chain, operational monitoring of control measures at every stage, and management plans that spell out what to do during both normal operations and emergencies.
Microbiological Standards
The microbiological standard is the strictest and simplest: E. coli or thermotolerant coliform bacteria must not be detectable in any 100 ml sample of treated drinking water. There is no acceptable low level. Any detection at all indicates fecal contamination and a potential pathway for waterborne diseases like cholera, typhoid, and dysentery. This zero-tolerance standard applies to all treated water supplies, regardless of the country or the size of the system.
WHO considers microbial contamination the single greatest risk to drinking water safety. Disinfection, typically with chlorine, is the primary barrier. The guidelines emphasize that effective disinfection should never be compromised to reduce chemical by-products, because the immediate risk of infectious disease outweighs the long-term risk from low levels of disinfection chemicals.
Chemical Contaminant Limits
WHO sets guideline values for dozens of inorganic and organic chemicals. A few of the most widely relevant limits illustrate how these work in practice.
Metals and Inorganic Chemicals
Arsenic is one of the most significant drinking water contaminants globally, particularly in parts of South and Southeast Asia where it occurs naturally in groundwater. The WHO guideline value is 0.01 mg/L (10 parts per billion). Long-term exposure above this level increases the risk of skin lesions, cardiovascular disease, and several cancers. Lead has no safe threshold. WHO treats it as a contaminant where exposure should be minimized as far as practically possible, with an action-triggering level of 0.01 mg/L. Mercury is capped at 0.002 mg/L.
Nitrate and Nitrite
Nitrate and nitrite are common in agricultural areas where fertilizer runoff enters groundwater. The WHO guideline sets nitrate at 50 mg/L (measured as nitrate ion) and nitrite at 3 mg/L (measured as nitrite ion). These limits are specifically designed to protect bottle-fed infants, who are the most vulnerable group. In infants, high nitrate levels interfere with the blood’s ability to carry oxygen, a condition called “blue baby syndrome” that can be life-threatening. Because the limits are set to protect infants, they also provide a wide safety margin for adults and older children.
Organic Chemicals
Benzene, a known carcinogen found in industrial pollution and fuel contamination, has a guideline value of 10 µg/L (micrograms per liter). WHO calculates this based on an acceptable excess lifetime cancer risk of one in 100,000. Pesticides, solvents, and other synthetic organic compounds each have individual guideline values, generally set using the same risk-based approach: estimating how much a person could consume daily over a lifetime without appreciable health effects.
Disinfection By-products
When chlorine or other disinfectants react with natural organic matter in water, they create by-products. The most common group is trihalomethanes, which form when chlorine interacts with decomposing plant material. Bromate can form when water containing bromide is treated with ozone. WHO sets individual guideline values for these compounds, but the guidelines are clear about the trade-off: the health risks from inadequate disinfection are far more immediate and severe than the risks from by-products at typical concentrations. Water suppliers should control by-products through better source water management and optimized treatment, not by reducing disinfection.
Radiological Screening Levels
For radioactive contamination, the WHO guidelines use a two-tier approach. The overarching limit is an individual dose of 0.1 millisieverts per year from drinking water. To make this practical for water utilities, WHO provides screening levels: 0.5 becquerels per liter for gross alpha activity and 1 becquerel per liter for gross beta activity. If a water sample falls below both screening levels, no further testing is needed. If either level is exceeded, the utility must identify the specific radioactive substances present and assess whether the annual dose limit is actually being breached.
Taste, Odor, and Appearance
Some water quality issues are not directly dangerous but affect whether people are willing to drink the water. WHO addresses these through aesthetic guidelines. While WHO does not publish binding numbers for these parameters, national standards commonly target a pH between 6.5 and 8.5, total dissolved solids below 500 mg/L, and minimal color and odor. Water outside these ranges may taste metallic, salty, or unpleasant, pushing people toward less safe alternative sources. For this reason, aesthetic quality is considered part of overall water safety even though the contaminants involved are not typically toxic at the levels encountered.
How Countries Use WHO Guidelines
No country is required to adopt WHO guideline values directly. The guidelines are explicitly designed to be adapted. A country with naturally high fluoride in its groundwater, for instance, may need to set a stricter national standard and invest in removal technology. A country where arsenic is not naturally present may focus regulatory resources elsewhere. WHO encourages each nation to conduct its own risk assessment, prioritize the contaminants most relevant to its population, and set standards that are achievable with available technology and infrastructure.
In practice, high-income countries often set standards that are equal to or stricter than WHO guidelines. The U.S. EPA, for example, sets its arsenic standard at the same 0.01 mg/L. The European Union has recently tightened lead limits below the WHO value. Lower-income countries may adopt WHO values directly or, in some cases, set less stringent interim targets as they build treatment capacity, with the understanding that they will tighten standards over time.
The Water Safety Plan framework has been adopted in over 90 countries as of the most recent WHO reporting. This shift from end-point testing to proactive risk management represents the most significant change in global drinking water safety practice over the past two decades. Rather than discovering contamination after it reaches consumers, the goal is to prevent it from entering the supply in the first place.