Researchers tested breast milk and fingernails from 74 nursing mothers to measure how much mercury they were passing to their babies. Mercury is a toxic metal found in the environment and some foods. The study found that while mercury levels in breast milk were relatively low, they were still high enough to potentially concern doctors. Factors like the mother’s age, how many children she had, and her dental fillings affected mercury levels. The good news is that breastfeeding is still recommended as safe and healthy, but mothers should be aware of mercury sources and try to limit their exposure when possible.

The Quick Take

  • What they studied: Whether mercury from the environment passes into breast milk and could harm nursing babies
  • Who participated: 74 mothers who were actively breastfeeding, recruited from health centers in urban areas
  • Key finding: Mercury was detected in all breast milk samples, and the amounts were high enough that health experts consider them a potential concern for babies, even though the levels weren’t extremely high
  • What it means for you: If you’re breastfeeding, you don’t need to stop—breastfeeding is still very beneficial. However, being aware of mercury sources (like certain fish) and trying to limit your exposure may help reduce what passes to your baby

The Research Details

Scientists collected breast milk samples and fingernail clippings from 74 nursing mothers and measured the amount of mercury in each sample using a special laboratory machine called atomic absorption spectrophotometry. This machine can detect tiny amounts of mercury. The mothers also answered detailed questions about their diet, their jobs, their health history (including dental work), and information about their babies. By comparing the mercury levels with all this information, researchers could figure out which factors made mercury levels higher or lower.

Fingernails were tested because they act like a record of what’s been in your body over time—similar to how tree rings show a tree’s age. Breast milk was tested because that’s the direct route mercury takes to babies. The researchers then calculated whether the amount of mercury babies were getting through breast milk could cause health problems.

This research approach is important because it looks at mercury exposure in a real-world situation—nursing mothers and their babies—rather than just in a lab. By measuring mercury in two different places (nails and milk), scientists could understand both long-term exposure and what’s actually being passed to babies right now. The study also looked at many different factors that might affect mercury levels, which helps identify who might be at higher risk.

This study has some strengths: it measured mercury using a reliable scientific method, included a reasonable number of participants, and looked at multiple factors. However, the study was relatively small (74 mothers), and it was done in one location, so results might not apply everywhere. The study shows what’s happening at one point in time but doesn’t follow mothers and babies over months or years to see if mercury actually caused health problems.

What the Results Show

Mercury was found in every single breast milk sample tested. The amounts ranged from 0.60 to 1.90 micrograms per liter (a very tiny measurement). Interestingly, mercury levels in fingernails were much more variable, ranging from 0.30 to 9.71 micrograms per gram, suggesting that some mothers had much higher overall mercury exposure than others.

When researchers calculated how much mercury babies were getting each day through breast milk, they found it exceeded the safe level that health experts recommend (called the reference dose). This means that based on the amounts found, babies were potentially getting more mercury than what’s considered completely safe. However, it’s important to note that ’exceeding the safe level’ doesn’t automatically mean babies will get sick—it means there’s a potential concern that warrants attention.

Surprisingly, the study found that eating fish, taking vitamins, or the mother’s job didn’t significantly affect mercury levels in breast milk. However, several other factors did matter: older mothers had higher mercury in their milk, mothers with more children had different levels, mothers with dental fillings had higher levels, and babies who weighed less had mothers with higher mercury in their milk.

The study found that longer pregnancies were associated with higher mercury in breast milk, and babies with lower body weight had mothers with higher mercury levels. These connections suggest that mercury exposure might affect fetal development or that smaller babies are more vulnerable to mercury’s effects. The research also identified that fruit consumption and the number of children a mother had were important factors in predicting mercury levels.

This research adds to growing evidence that mercury is a widespread environmental concern affecting nursing mothers and babies. Previous studies have shown mercury in breast milk, but this study is notable for measuring it in a specific population and identifying the particular factors that influence levels. The finding that mercury exceeds safe reference doses aligns with other research suggesting that environmental mercury exposure is a public health concern, particularly for vulnerable populations like infants.

The study only included 74 mothers from urban health centers, so results might not apply to mothers in rural areas or other countries. The study was done at one point in time, so it doesn’t show whether mercury levels change over time or whether babies actually developed health problems. The study didn’t measure mercury in the babies themselves, only what was in the milk. Additionally, the study couldn’t prove that specific sources caused the mercury—it only showed associations between factors and mercury levels.

The Bottom Line

Continue breastfeeding—the benefits are well-established and important for baby health. However, try to limit mercury exposure by: being cautious with high-mercury fish (like shark, swordfish, and king mackerel), eating a varied diet with fruits and vegetables, and discussing dental fillings with your dentist if you’re planning pregnancy or currently nursing. If you have concerns about mercury exposure, talk to your doctor about testing. (Confidence level: Moderate—based on this single study plus existing research)

All breastfeeding mothers should be aware of this information, particularly those who eat a lot of fish, live in areas with environmental pollution, or have had recent dental work. Mothers of babies with low birth weight may want to discuss mercury exposure with their pediatrician. This information is less relevant for mothers who formula-feed, though they should still be aware of mercury in their own diet.

Mercury accumulates in the body over time, so reducing exposure now may help lower levels in future pregnancies and breastfeeding. You won’t see immediate changes, but consistent efforts to limit mercury sources over weeks and months should gradually reduce your exposure and what passes to your baby.

Want to Apply This Research?

  • Log fish consumption weekly, noting the type of fish eaten and portion size. Track this against any symptoms or concerns you notice in yourself or your baby to identify patterns.
  • Set a weekly reminder to check a ‘safe fish’ list before meal planning. Aim to eat high-mercury fish no more than once per month and choose low-mercury options (like salmon, sardines, and canned light tuna) for other meals.
  • Create a monthly summary of fish consumption and dietary patterns. If you’re concerned about mercury exposure, work with your doctor to establish a monitoring plan, which might include periodic testing or dietary adjustments over 3-6 month periods.

This research suggests a potential health concern regarding mercury in breast milk, but it does not mean you should stop breastfeeding. Breastfeeding provides essential nutrients and protection for babies and remains the recommended feeding method. If you have specific concerns about mercury exposure or your baby’s health, please consult with your healthcare provider, pediatrician, or a registered dietitian. This article is for educational purposes and should not replace professional medical advice. The findings are from a single study and should be considered alongside other research and expert recommendations from organizations like the CDC and WHO.