Exposure via breast milk refers to the transfer of substances—including medications, environmental chemicals, or infectious agents—from a breastfeeding mother to her infant through breast milk. While breast milk is universally recognized as the best source of nutrition for babies, understanding what can pass through it and how to minimize potential risks is important for protecting infant health while preserving the many benefits of breastfeeding.

Understanding How Substances Enter Breast Milk

Almost any medicine or chemical that enters a mother’s bloodstream can make its way into breast milk, though usually in very small amounts. Substances reach breast milk through a process called diffusion, which means they move from the mother’s blood into the milk naturally. The amount that transfers depends on several factors, including how much of the substance is in the mother’s blood and the specific properties of that substance^[5].

In general, less than one percent of a medication taken by a breastfeeding mother will pass through to the baby through breast milk. Modern technology can detect extremely tiny amounts of substances in milk, but finding traces of something does not automatically mean it will cause harm to the baby^[7].

Substances that are fat-soluble—meaning they dissolve in fat rather than water—are more likely to accumulate in breast milk because human milk contains significant amounts of fat. This is particularly true for persistent chemicals that the mother’s body has stored in fat tissue over time. During breastfeeding, when the body mobilizes these fat stores to produce milk, these stored substances can be released^[14].

Environmental and Occupational Chemical Exposures

Breastfeeding mothers may encounter various chemicals in their everyday surroundings or through their work. These exposures deserve attention, though health experts consistently emphasize that for most common chemicals, the benefits of breastfeeding far outweigh the potential risks from exposure through breast milk^[1][2].

Per- and polyfluoroalkyl substances, commonly known as PFAS, are synthetic chemicals used in many consumer products including non-stick cookware, water-repellent clothing, stain-resistant fabrics, and certain cosmetics. Babies can be exposed to PFAS while breastfeeding because these chemicals can transfer into milk. However, based on current scientific understanding, the benefits of breastfeeding still outweigh any potential risks from PFAS exposure. Mothers with concerns about PFAS should discuss them with their healthcare provider^[1].

Pesticides are chemicals designed to destroy unwanted plants, insects, rodents, or fungi. People can be exposed to pesticides by breathing them in, eating foods sprayed with them, or absorbing them through the skin. Some pesticides may pass into breast milk, but research on the effects of this exposure is limited, so safe exposure levels are not well established. Breastfeeding mothers should try to reduce their exposure to pesticides as much as possible^[1].

Workplace exposures represent another potential route for chemicals to enter breast milk. Women who work in healthcare facilities may encounter chemotherapy drugs, which are designed to damage or kill cells even at very low doses. Some of these drugs can cause cancer and have no known safe exposure level. Similarly, women working in dry cleaning facilities may be exposed to perchloroethylene (PCE), a solvent that studies show can enter breast milk when mothers are exposed to it. Higher and longer exposures lead to greater amounts in milk^[2].

Laboratory workers face potential exposure to various chemicals and infectious agents including solvents and formaldehyde. Some of these chemicals can enter breast milk. Workers in these environments should follow recommended safety practices, use appropriate safety cabinets, and discuss with their healthcare provider and employer how to reduce exposure as much as possible^[2][11].

Lead Exposure and Breastfeeding

Lead is a toxic metal that can have serious effects on health, particularly for developing children. Even low levels of lead in blood can negatively affect a child’s intelligence, ability to pay attention, and academic achievement. No safe blood lead level has been identified for children^[13].

Mothers can expose their babies to lead through breast milk in two ways. First, if a mother is directly exposed to lead during pregnancy or while breastfeeding, it can pass through her milk. Second, lead that has been stored in the mother’s bones and teeth from past exposures can be released into her bloodstream during pregnancy and breastfeeding, then transferred to the baby^[13].

Breastfeeding recommendations vary based on the mother’s blood lead level, measured in micrograms per deciliter. Women with blood lead levels between 5 and 39 micrograms per deciliter can continue breastfeeding, but their baby’s blood lead levels should be monitored. If a mother’s blood lead level reaches 40 micrograms per deciliter or higher, she is encouraged to pump and discard her milk until her levels drop below that threshold^[13].

Medications and Breast Milk Safety

Most medications are safe to take while breastfeeding, and taking medicine does not usually mean a mother needs to stop nursing temporarily or permanently. The number of adverse reactions to drugs passing through breast milk is actually quite small^[7][10].

Medications enter breast milk through the mother’s blood, so milk concentrations depend on how much medication is in her bloodstream and how easily that particular drug diffuses into milk. Unlike during pregnancy when substances cross the placenta directly, during breastfeeding the baby must absorb the medication through their digestive system after consuming the milk^[10].

Several factors affect how medications in breast milk might impact a baby. The infant’s age and health play important roles—premature babies, newborns, and babies with kidney or other health problems face the highest risk from medication exposure. In contrast, healthy babies who are six months or older face the lowest risk because their bodies are better able to process medications. The amount of breast milk consumed also matters—a two-week-old baby who is exclusively breastfed consumes much more milk daily than a nine-month-old who nurses just once or twice a day^[7][10].

Only a few medications are truly incompatible with breastfeeding. These include certain anticancer drugs, lithium, oral retinoids, high doses of iodine, amiodarone, gold salts, and some radiopharmaceuticals. For most other medications, safer alternatives exist or the medication itself poses minimal risk to the nursing infant^[12].

Drug manufacturers are not required to provide clinical data on medication safety during breastfeeding when seeking approval to market their products, as it would be unethical to expose infants to potential harm through research. This means most medications are unlicensed for use during breastfeeding, even when available data suggests they are safe. Package inserts may warn against use while breastfeeding simply due to lack of formal testing, not because of known dangers^[7].

Transmission of Infectious Diseases

A large body of evidence demonstrates both the protective effects of breastfeeding against infections and the documented transmission of specific infections to infants through breast milk. The fear and anxiety surrounding infectious diseases in breastfeeding mothers can sometimes lead healthcare providers to recommend stopping breastfeeding unnecessarily, which deprives the infant of important benefits exactly when they are most needed^[3][9].

Confirming that an infection was transmitted through breastfeeding requires several steps. First, the infectious agent must be identified in the mother’s milk or breast tissue. Then, the infant must develop a clinically significant infection caused by the same agent. Other possible routes of transmission—such as airborne exposure, contact with other infectious fluids, or transmission before or during birth—must be ruled out. This process of confirmation can be quite difficult^[3][9].

Infectious organisms can reach breast milk either through secretion into the fluid or cellular components of milk or through contamination during or after milk expression. Decisions about breastfeeding when a mother has an infectious illness should balance the potential benefits of breastfeeding against the known or estimated risk of the infant acquiring a significant infection and how severe that infection might be^[3][9].

Despite concerns about disease transmission, few illnesses actually pass through breast milk. The unique protective properties of breast milk help protect infants from colds and other typical childhood viruses. When a breast milk mix-up occurs—such as when a child accidentally receives another mother’s expressed milk at a childcare facility—the risk of transmitting an infectious disease is very small^[8].

For mothers living with HIV who are on antiretroviral therapy with a sustained undetectable viral load, the risk of transmission through breastfeeding is less than one percent. The risk from exposure to hepatitis B or C through breast milk is also very low, as these viruses cannot spread through breastfeeding or close contact unless there is exposure to blood^[8].

Special Circumstances and Medical Imaging

Contrast materials used for computed tomography (CT) or magnetic resonance imaging (MRI) scans are not concerning during breastfeeding. These substances pass into milk in such small amounts that they pose no risk to nursing infants. Mothers do not need to interrupt breastfeeding after receiving contrast for these imaging procedures^[8].

However, the use of radiopharmaceuticals—radioactive substances used in certain medical procedures—requires different precautions. Substances like radioactive iodine 131 can accumulate in the lactating breast and increase risk to the infant. If a mother requires treatment with radioactive iodine 131, breastfeeding should be stopped^[10][12].

Chemical and Biological Threats in Emergency Situations

In today’s world, there is a growing risk of exposure to chemical and biological weapons, particularly in conflict zones and areas affected by terrorism. Despite international conventions banning these weapons, state-sponsored use on vulnerable populations has occurred, as has use by individuals and non-state actors. Women of reproductive age and young children, who often represent the most vulnerable populations, face increased likelihood of exposure^[4].

Children aged zero to five years carry the most significant burden of conflict-related deaths of all age groups. Women of reproductive age living in conflict zones have three times higher mortality than women in predominantly conflict-free settings. Despite this stark evidence, guidance specifically addressing whether women can safely continue breastfeeding after chemical or biological exposures has historically been limited and difficult to access^[4].

Current information about breastfeeding safety following chemical or biological events is scattered across different sources and often not written in easily understandable language for mothers or first responders. Guidelines from health agencies are typically specific to individual agents, and information about safety for particular populations or situations can be ambiguous and not readily accessible when decisions need to be made quickly^[4].

Minimizing Infant Exposure

Several practical strategies can help minimize infant exposure to potentially harmful substances in breast milk while allowing mothers to continue providing this optimal nutrition. For medications taken once daily, mothers can time the dose after breastfeeding and before the infant’s longest sleep interval. This allows more time for the medication to clear from the mother’s bloodstream before the next feeding^[10].

Medications with certain characteristics are safest for breastfeeding mothers. These include drugs that bind strongly to proteins in the blood, leaving less free drug available to enter milk. Medications with low milk-to-plasma ratios are preferable because less drug reaches the milk. Those with shorter half-lives clear from the body more quickly, reducing exposure time^[7].

To reduce environmental chemical exposures, breastfeeding mothers can take several practical steps. These include avoiding smoking, recreational drugs, and alcohol; reducing intake of animal fats while increasing consumption of grains, fruits, and vegetables; washing or peeling fruits and vegetables to remove pesticides; and limiting contact with common household chemicals like solvents in paints, non-water-based glues, and nail polish. Avoiding breathing smoke and fumes from fires and removing work clothes and shoes carefully to prevent bringing toxic chemicals into the home are also important^[15][16].

The Protective Role of Breast Milk

Despite concerns about harmful substances, breast milk itself provides significant protection for babies. Human milk contains antioxidants and immune protective factors that can actually lessen the effects of environmental toxins that babies are exposed to, whether in the womb or in their environment after birth^[15].

Environmental pollutants have a greater impact on babies while they are developing inside their mother’s womb during pregnancy than after birth through breastfeeding. It is during these critical developmental stages that toxins can be most damaging. After birth, breastfeeding becomes the baby’s best protection for normal growth and development and can help limit damage caused by exposure to toxins before birth^[15].

The advantages of breastfeeding for both mother and baby far outweigh any risks from common toxins that are sometimes found in breast milk. Breast milk provides the most complete form of nutrition for infants and protects against a variety of diseases and conditions. It also benefits mothers by decreasing postpartum bleeding and reducing the risk of breast and ovarian cancers^[6].