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Understanding Drinking Water Standards

This article explains the drinking water standards developed to protect public health and how public drinking water utilities ensure our tap water is safe.

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Updated:

July 24, 2025

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Drinking water sources, including surface water (rivers, lakes, reservoirs) and groundwater (wells, springs), can be polluted through various natural processes and human activities. Pollutants can include debris, bacteria, viruses, radionuclides, and dissolved chemicals that can make the water unsafe for human consumption without treatment. Drinking water standards are regulations and guidelines that provide safety thresholds for different pollutants in public drinking water. The standards are created to (i) protect public health and (ii) ensure that drinking water has acceptable taste, smell, and color.

The Safe Drinking Water Act (SDWA) is a US federal law enacted in 1974 to protect the quality of public drinking water in the United States. The US Environmental Protection Agency (EPA) sets national drinking water standards under SDWA that are then enforced by states. In Pennsylvania, the national drinking water standards are enforced by the Pennsylvania Department of Environmental Protection (PA DEP). Based on local water quality concerns, states can set and enforce rules that are stricter than the federal drinking water standards. State agencies can additionally regulate and enforce standards on pollutants that have not yet been regulated at the federal level. If the US EPA and a state agency set different drinking water standards for the same contaminant, public water utilities must meet the lower or stricter standard.

Public versus Private Water Systems

National and state drinking water standards only apply to public water systems. A public water system has at least 15 service connections and provides water for human consumption to a minimum of 25 people for at least 60 days a year. Public water systems are sometimes referred to as "city water," although they may be publicly or privately owned and managed. Because they are regulated, public water systems have to monitor their drinking water sources (groundwater or surface water), treat the water to remove any pollutants, and distribute the treated water to consumers through a series of underground pipes or service connections. Water reaching a customer's tap must meet all national and state (if any) drinking water standards. Public water systems can be classified into three main categories:

Community water systems that provide water to residential areas often to the same populations year-round.
Transient non-community water systems that provide water only for a short period of time to people in areas such as campgrounds, gas stations, resorts, etc.
Non-transient non-community water systems that are used in facilities with their own water systems and serve the same people for at least six months of the year. They are non-transient since they serve the same people for extended periods of time in areas such as schools, hospitals, office buildings, etc.

Private water systems are not regulated under the SDWA because they serve fewer than 25 people or have fewer than 15 service connections. They commonly serve single residential units in communities that are located outside the public water system service areas. Private water systems can be serviced by on-site drilled or hand-dug wells, springs, or cisterns. Because private water systems are not regulated, the owner or user is voluntarily responsible for protecting the water source to prevent pollution, testing water quality, and implementing treatment to improve water quality. Although they are not required to, private water system users are encouraged to ensure that their water quality meets national and state drinking water standards to protect their health.

Commercially Bottled Water

Commercially bottled water is regulated by the US Food and Drug Administration (FDA). Bottled water can be sourced from public water systems or groundwater sources such as wells and springs. FDA regulations set guidelines on the quality of the water and requirements for labeling and manufacturing practices. Bottled water quality standards generally follow the same drinking water standards used by the EPA for public water supplies. However, water testing and reporting requirements are less frequent and less transparent than public tap water. Learn more about bottled water here: Understanding Bottled Water.

How Public Drinking Water Standards are Established

The SDWA requires the US EPA to review and revise existing standards as well as establish new standards for unregulated contaminants based on current knowledge and technologies to improve public health protection. Establishing new drinking water standards is a multi-step and science-driven process that can take several years. The steps are available at Understanding How EPA Develops a New Drinking Water Regulation and are also summarized below:

Step 1: Identification of contaminants

This step involves identifying contaminants that are not regulated in drinking water through the Contaminant Candidate List (CCL). Published every five years, the CCL provides a list of unregulated contaminants known or anticipated to occur in public water systems that may pose public health risks. The CCL does not impose any requirements on public water systems but provides a list of priority contaminants that are then evaluated for potential regulations. After a final CCL is published, EPA conducts additional research and collects data to evaluate the listed contaminants. This data collection process helps EPA determine whether or not to regulate at least five contaminants from the CCL in a separate process called Regulatory Determinations. The CCL is also used to determine contaminants that public water systems are required to monitor as part of the Unregulated Contaminant Rule (UCMR) every five years. For example, per- and polyfluoroalkyl substances (PFAS) were included in the CCL 5 in 2022 and were also included in UCMR 5 (2022-2026).

Step 2: Evaluation

In this step, the US EPA uses existing and newly collected scientific research and data to determine whether the rulemaking process to establish national primary drinking water standards for specific contaminants in the CCL is warranted. The following criteria are used: (i) the contaminants are found to pose adverse effects on human health, (ii) they occur at high frequencies and at levels of public health concern in drinking water supply, and (iii) regulation can meaningfully reduce human health risks. The evaluation report is published as a preliminary regulatory determination report available for public commentary.

Step 3: Regulation

If a decision to regulate any contaminant in the CCL is reached, the EPA will start the rulemaking process to establish the national primary drinking water regulations (NPDWR). Using the available scientific research and monitoring data on human health effects, EPA establishes a maximum contaminant level goal (MCLG). This is the safest level of a contaminant in drinking water at which there are no known or expected health risks even to sensitive populations such as children, the elderly, or individuals with compromised immune systems. For pathogens that can make people sick (e.g., parasites, bacteria) and potentially carcinogenic chemicals (e.g., arsenic, lead), the MCLG is set as zero if there is no dose below which the contaminants can be considered safe. MCLGs are non-enforceable, but can be used as guiding benchmarks.

After establishing the MCLG, EPA sets an enforceable drinking water standard that is called the maximum contaminant level (MCL). MCLs are the highest concentrations of contaminants legally allowed in drinking water from a public water system. MCLs are to be set as close to the MCLGs as is feasible. The selection of MCLs is based on available treatment technologies, the costs of treatment, and the availability of analytical tools and methods to assess compliance. MCLs can be adjusted if the benefits of a new MCL does not justify the costs following an economic analysis. Public water systems must comply with new MCLs approximately three years after they are finalized. This period allows utilities to conduct monitoring and implement needed treatment technologies and plans to comply with the new MCLs. EPA also reviews all drinking water standards every six years to determine whether changes are needed. Learn more at How EPA Regulates Drinking Water Contaminants.

If a decision to not regulate is made for contaminants in CCL, EPA may take no additional action or develop a health advisory level. Health advisory levels identify the concentration of a contaminant in drinking water at which there are no adverse health effects and/or aesthetic effects over specific exposure durations (e.g., 1 day, 10 days, a lifetime). Health advisory levels are also typically developed for unregulated contaminants during response to emergency contamination situations such as spills. Health advisory levels are not legally enforced but are used as drinking water safety benchmarks by public water systems and health officials. Many contaminants of emerging concern start by having health advisory levels as more research is done before MCLs are set. Learn more at Drinking Water Health Advisories.

Types of Drinking Water Standards

There are two categories of drinking water standards:

National primary drinking water regulations (NPDWR or primary standard): These are legally enforceable standards that apply to public water systems. Primary standards limit the levels of specific contaminants that can adversely affect public health and are known or anticipated to occur in water from public water systems. Most contaminants with primary standards have maximum contaminant levels (MCL). MCLs are the highest concentration of contaminants allowed in drinking water delivered to a consumer. When present above the MCLs in drinking water, these contaminants can cause illnesses or chronic health effects. The US EPA has set legally enforceable primary drinking water standards for approximately 90 contaminants in drinking water. They include microorganisms like parasites and bacteria, disinfectants used to kill microbes (e.g., chlorine), disinfection byproducts that form when disinfectants react with organic matter in water, inorganic chemicals like heavy metals, organic chemicals like pesticides and some PFAS, and radioactive elements. These contaminants do not change the taste, color, or smell of drinking water. To access the full list of contaminants with primary standards, visit: National Primary Drinking Water Regulations. Apart from the concentration MCLs, enforceable primary standards can also be implemented as Hazard Index or Treatment Techniques:
1. Hazard Index MCLs: The hazard index is often used to evaluate human health risks from more than one chemical in drinking water, where the MCLs are set at 1.0 (unitless). It is calculated by dividing measured concentrations in drinking water by the corresponding contaminant's health-based threshold, which is the highest level determined not to pose human health effects. The numbers obtained after dividing are added together for all the chemical mixtures. If the number is greater than 1, then it is failing the MCL, and the MCL is met if it is less than 1. This method was used in 2024 to provide MCLs for some PFAS, such as PFNA, PFHxS, PFBS, and HFPO-DA (GenX chemicals).
2. Treatment technique (TT) rule: When an MCL is difficult to achieve, EPA implements a TT rule, which is an enforceable procedure or process intended to reduce the level of a contaminant in drinking water. This rule was implemented in the lead and copper rule because contamination originates from the corrosion of plumbing materials belonging to customers. This rule requires utilities to optimize treatment plant processes for corrosion control and monitor lead levels in distribution, with additional actions triggered if levels exceed an action level. For example, if more than 10% of tap water samples exceed the 15 µg/L action level for lead, the utility has to take action such as replacing some lead service lines and take additional steps to control the corrosivity of water.
National secondary drinking water regulations (NSDWR or secondary standard): These are non-enforceable guidelines or recommendations for pollutants that can negatively affect the aesthetic characteristics of drinking water (such as taste, odor, or color) or result in cosmetic effects such as skin or tooth discoloration. Although they do not pose health concerns, these contaminants can result in a metallic or salty taste in water or discolor water fixtures, laundry, skin, and teeth. The US EPA has therefore established secondary maximum contaminant levels (SMCLs) for 15 contaminants as guidelines for public water supplies to maintain the aesthetic appeal of drinking water. To access the full list of contaminants with secondary standards, visit: Secondary Drinking Water Standards: Guidance for Nuisance Chemicals.

Units of Measure

Apart from the Hazard Index and the Treatment Technique rules, enforceable drinking water standards (i.e., MCLs) and non-enforceable guidelines (i.e., MCLGs and Health Advisories) are often expressed as concentrations, which are the amount of contaminant present per unit volume of water. Different units can be used based on the contaminant characteristics and how they affect human health.

Radioactive contaminants are measured in picocuries per unit volume of water, e.g., pCi/L.
Pathogens like bacteria and protozoa are measured by counting the number of organisms per unit volume of water. For example, bacteria and Giardia can be measured as colonies per mL and cysts/L, respectively.
Dissolved chemicals are measured in mass per unit volume of water, such as milligrams per liter (mg/L), micrograms per liter (µg/L), and nanograms per liter (ng/L). Contaminants that present human health effects at low concentrations tend to have units for their MCLs, MCLGs, or health advisory levels set at ng/L or µg/L. These units can be converted from one to another using simple multiplication or division factors, as shown in Table 1 below. MCLs posted on the US EPA website are often converted to mg/L for uniformity reasons. For instance, Dioxin’s MCL is published as 0.00000003 mg/L, which is equivalent to 0.03 ng/L.

Table 1. Drinking water units and conversions
Unit	Example contaminants	Conversions
Picocuries per Liter (pCi/L)	Radionuclides such as radium	1 pCi/L = 0.001 nanocuries per liter (nCi/L)
Milligrams per liter (mg/L) or part per million (ppm)	Nitrate, disinfectants, and some metals	1 mg/L = 1 ppm 1 mg/L = 1000 µg/L or ppb 1 mg/L = 1000,000,000 ng/L or ppt
Micrograms per liter (µg/L) or parts per billion (ppb)	Trace metals such as lead and arsenic	1 µg/L = 1 ppb 1 µg/L = 0.001 mg/L or ppm 1 µg/L = 1000 ng/L or ppt
Nanograms per liter (ng/L) or parts per trillion (ppt)	PFAS, dioxin	1 ng/L =1 ppt 1 ng/L = 0.001 µg/L or ppb 1 ng/L = 0.000001 mg/L or ppm

How Public Water Systems Ensure the Safety of Your Tap Water

Because drinking water can be vulnerable to contamination, public water systems use a multi-barrier approach to protect public health. The multi-barrier approach simply refers to the several layers of action and initiatives that public water systems implement to protect the quality of water in consumers' taps. Although public water systems bear the responsibility of delivering safe drinking water to you as a customer, they do so in a shared effort between the utility and the community as follows:

Approach 1 – source water protection: Support from community members plays a crucial role by limiting contamination of drinking water sources. Utilities monitor their drinking water sources to assess the risk of contamination and also know water quality conditions.

Approach 2 – water treatment: Depending on the water source and the types of contaminants present, water will undergo multiple treatment processes that are designed to remove or inactivate contaminants present in source water. Treatment ensures that final product water meets or surpasses state and federal drinking water standards.

Approach 3 – monitoring and compliance: Utilities regularly conduct water quality monitoring from the source, after each treatment step, in storage tanks, and in the distribution systems. Monitoring goals include collecting data on contaminants present, performance of treatment systems, and evaluating any water quality deterioration as it is distributed to customers. Through testing, utilities are able to detect and address concerns promptly and ensure product water meets or surpasses state and federal drinking water standards. Monitoring data is submitted to state agencies.

Approach 4 – consumer awareness: Community water systems are required to provide customers with a water quality report called a Consumer Confidence Report (CCR) at least annually. This report may be mailed to customers or posted on the water company’s website. Customers who do not pay their own water bills (e.g., renters) may have to work with building managers or the utility to access CCRs. CCRs share with customers the water quality of the source water and the treated drinking water delivered to their taps. This report should also contain any instances where the utility violated any of the primary drinking water standards and how they are fixing the problem. Because contaminants with secondary standards are not enforced, the data may not be included in the CCR. Learn more at CCR Information for Consumers.

Drinking Water Advisories

Water utilities have a responsibility to provide safe drinking water. When drinking water may be unsafe as a result of contamination, treatment issues, or system failures, utilities or health officials may issue an advisory. A drinking water advisory is an official public notice alerting you that there's a problem with public drinking water and detailing actions you can take to avoid getting sick. Advisories are issued when there is an 'event' at the drinking water source, at the treatment plant, or within the distribution system that could degrade the quality of water delivered to your tap. Some common reasons or events can include:

Water main break or pipe damage: When water mains or pipes are damaged from leaks, cracks, or bursts that allow dirt, bacteria, or chemicals to contaminate water delivered to your tap.

Natural disasters such as flooding: Advisories may be issued during or after flooding events. Floodwater can contaminate water supplies, damage distribution pipes, or result in power outages that disrupt water treatment or distribution systems. Contaminants of concern during floods are dirt, bacteria, or chemicals.

Harmful algal blooms (HABs): Blooms from toxin-producing cyanobacteria in lakes, reservoirs, or rivers can be problematic for utilities. Water treatment processes can typically remove cyanobacteria cells and toxins. However, severe blooms can result in elevated toxin concentrations in treated water.

Violation of treatment technique requirements: Advisories may be issued if there is a breakdown in treatment technique requirements for pathogenic bacteria, viruses, and protozoan cysts. Learn more about violations here: PA DEP Public Notification.

Utilities work with local enforcing agencies (e.g., PA DEP) when advisories are needed and can communicate them by posting on websites, customer portals, and local news channels or direct communications to customers. You may notice changes in the color, smell, or taste during an advisory period. In some advisories, however, there may not be any observable impacts on tap water, but it is still recommended to follow guidance to protect your health. Public water systems take actions immediately to rectify or manage the 'events' resulting in an advisory. As corrective measures are being implemented, customers are urged to follow utility guidance messages. Tap water is often safe for consumption once the drinking water advisory is lifted. Table 2 below describes the different types of advisories:

Table 2. Common types of drinking water advisories
Advisory Type	What It Means	What You Should Do
Boil Water Advisory	Boil tap water before use. Possible contamination by germs that can make you sick.	Bring water to a rolling boil for 1-3 minutes and allow to cool before use. Use boiled water or commercially bottled water for drinking, cooking, making ice, brushing teeth, preparing baby formula, and caring for pets.
Do Not Drink Advisory	Water is unsafe to consume. Possible chemical or toxin contamination.	Use commercially bottled water for drinking, cooking, washing fruits and vegetables, making ice, brushing teeth, preparing baby formula, and caring for pets.
Do Not Use Advisory	Water is unsafe for all uses and contact.	Avoid any contact with water; follow utility instructions and use commercially bottled water for all drinking, cooking, bathing, or washing.

Advisories typically apply to public water system users only. However, private water systems users are encouraged to pay attention to local conditions, especially during natural disasters such as flooding. Learn more here: Post-Flood Drinking Water Safety for Private Water Systems.

Customer Feedback

The aesthetic properties of water, such as taste, smell, or color, can be very subjective but can also influence our perceptions of drinking water safety. Public water utilities work hard to ensure that the water delivered to you meets drinking water standards and is safe. If you notice unusual and sudden changes in tastes, odors, or colors in your tap water when there is no active advisory issued, it is worth investigating. Sometimes these issues may originate from household plumbing, the distribution lines, or even from the utility. You can start by checking in with others in your neighborhood and then notifying your utility. The utility can investigate to determine and address the cause.