Five contaminants routinely found in US tap water cross the placental barrier and reach a developing baby: lead, inorganic arsenic, mercury, PFOA, and hexavalent chromium (chromium-6). The placenta filters many toxins, but not these. Two of the five, lead and PFOA, have no safe level recognized for fetal exposure. Arsenic's health guideline is 0.004 ppb, which is 2,500 times below the EPA legal limit, and the OEHHA public health goal for chromium-6 is 0.02 ppb. A reverse-osmosis filter is the reliable way to reach these levels for the water you drink and cook with.
The placenta is often described as a shield. It is more like a selective gate. It blocks large or fat-soluble molecules well, but small metals, protein-binding chemicals, and substances that mimic essential minerals slip through. When they do, the fetus is exposed to concentrations close to what circulates in maternal blood, and its liver and kidneys are not finished building the enzymes needed to clear them.
This post walks through each of the five, what actually reaches the fetus, and the number that protects a developing baby rather than the number the law allows. It's national guidance, not tied to any one state.
Which Tap Water Contaminants Cross the Placenta?
The five below are the ones with the clearest evidence of placental transfer plus meaningful US tap-water occurrence. Lead crosses by passive diffusion with essentially no barrier (Bellinger, 2005). Arsenic reaches fetal concentrations mirroring maternal blood (Concha et al., 1998). PFOA is actively transported across the placenta and shows up in cord blood (ATSDR Toxicological Profile for Perfluoroalkyls, 2020).
The table compares each contaminant's EPA legal limit, the health-based guideline from an authority (OEHHA or EWG), and the CheckYourTap safe level calibrated for pregnancy. The last column is the health-protective figure, not the legal one.
| Contaminant | EPA Legal Limit | Health Guideline (Authority) | CheckYourTap Safe Level (Pregnancy) |
|---|---|---|---|
| Lead | 15 ppb action level (→10 ppb by 2027) | 0.2 ppb (EWG) | No safe level — target 0 |
| Arsenic | 10 ppb | 0.004 ppb (OEHHA) | 0.004 ppb (OEHHA, 2004) |
| Chromium-6 | No separate limit (100 ppb total chromium) | 0.02 ppb (OEHHA PHG) | 0.02 ppb (OEHHA, 2011) |
| PFOA | 4.0 ppt (MCL) | 0 (EPA MCLG) | No safe level — MCLG 0 |
| Mercury | 2 ppb | 1.2 ppb (OEHHA / EWG) | 0.4 ppb (pregnancy estimate) |
Two rows have no number we can honestly fill with a "safe" figure. For lead and PFOA, the health target for fetal exposure is zero. The other three carry health guidelines far below what a compliance report treats as passing.
Citation capsule: Lead crosses the placenta by passive diffusion with essentially no fetal barrier (Bellinger, 2005), and health agencies including OEHHA and the CDC recognize no safe level of lead exposure. The EPA's health-based goal (MCLG) for PFOA is zero (EPA PFAS regulation, 2024). Both contaminants therefore have no safe level for a developing baby.
Lead and PFOA: The Two With No Safe Level
Lead and PFOA share a hard truth: there is no concentration considered safe for a fetus. The EPA sets a lead action level of 15 ppb (being lowered to 10 ppb under the 2024 Lead and Copper Rule Improvements) and an enforceable PFOA limit of 4.0 ppt, but in 2024 it set the health-based goal for PFOA at zero. These are feasibility standards, not safety lines.
Pregnancy makes lead more dangerous, not less. Lead is chemically similar to calcium and is stored in bone over a lifetime. The bone remodeling that supplies calcium to the fetal skeleton also releases decades of stored lead back into the bloodstream, where it crosses the placenta freely (Bellinger, 2005). Its effects on fetal synapse formation and neurodevelopment are irreversible, which is why prevention is the only real strategy.
PFOA behaves differently but ends up in the same place. It binds tightly to proteins in maternal blood and is actively transported across the placenta, so cord blood carries measurable levels. Prenatal PFOA exposure is linked to lower birth weight, altered thyroid signaling, and a suppressed fetal immune response (EFSA, 2020; ATSDR Toxicological Profile for Perfluoroalkyls, 2020). Because it is a "forever chemical," it does not wash out between glasses of water.
Why Isn't the Legal Arsenic Limit a Health Limit?
The EPA set the arsenic limit at 10 ppb in 2001, but the National Research Council estimated lifetime exposure at that level carries a combined lung and bladder cancer risk near 1 in 300 (National Research Council, 2001). That is far above the agency's usual 1-in-10,000 ceiling. The 10 ppb limit balanced treatment cost against risk. It was never a declaration of safety.
The health science drew the line 2,500 times lower. The California Office of Environmental Health Hazard Assessment (OEHHA) set a public health goal of 0.004 ppb, the level tied to negligible added risk. In pregnancy the stakes sharpen: inorganic arsenic crosses the placenta at concentrations mirroring maternal blood, and the fetal liver lacks the enzymes to methylate and clear it (Concha et al., 1998). Exposure well below the legal limit is tied to lower birth weight and higher preterm birth risk (Rahman et al., 2009; ATSDR Toxicological Profile for Arsenic, 2007).
Citation capsule: The EPA's legal limit for arsenic is 10 ppb, while the OEHHA public health goal tied to negligible lifetime risk is 0.004 ppb (OEHHA, 2004). The legal limit is therefore 2,500 times higher than the health-protective guideline that matters during pregnancy.
How Do Chromium-6 and Mercury Affect a Developing Fetus?
Hexavalent chromium is the "Erin Brockovich" chemical, and it slips through a regulatory gap. The EPA has no separate federal limit for chromium-6; it regulates total chromium at 100 ppb, so chromium-6 can legally sit thousands of times above its health goal. The OEHHA public health goal is 0.02 ppb (OEHHA, 2011). Chromium-6 crosses the placenta and drives oxidative stress and apoptosis in fetal tissue, with early-pregnancy exposure disrupting fetal development (EPA chromium in drinking water; ATSDR Toxicological Profile for Chromium, 2012). Our vulnerability model for pregnancy lands slightly lower, near 0.017 ppb, but we publish the defensible 0.02 ppb OEHHA goal as the headline number.
Mercury in drinking water is mostly inorganic, and it too crosses the placenta to reach the fetus. The EPA legal limit is 2 ppb and the OEHHA and EWG health guideline is 1.2 ppb. Because a developing body clears inorganic mercury less efficiently than an adult, we set a stricter pregnancy estimate of 0.4 ppb (OEHHA Public Health Goals; EWG Tap Water Database). That 0.4 ppb figure is our vulnerability-adjusted estimate, not a published authority threshold. We treat it as a directional refinement of the 1.2 ppb health guideline, not a hard limit, because honest sourcing beats false precision.
What Actually Removes These Five From Tap Water?
Standard carbon pitcher and fridge filters, the Brita-and-PUR tier, are not built to reduce heavy metals, PFAS, or chromium-6 to the levels pregnancy calls for. Reaching these numbers takes the right technology matched to the contaminant.
- Reverse osmosis (RO) is the reliable default. It forces water through a semi-permeable membrane and removes up to ~99% of lead, arsenic, mercury, chromium-6, and PFOA in one step. An under-sink unit certified to NSF/ANSI Standard 58 is the gold standard for pregnancy. Because RO also strips beneficial minerals, keep calcium and magnesium up through diet or prenatal vitamins.
- Ion exchange and activated alumina target specific metals. Ion exchange resins certified to NSF/ANSI Standard 53 remove lead well; activated alumina is a strong point-of-entry option for high-arsenic wells.
- Certified carbon block rated to NSF/ANSI 53 or 58 can reduce lead and PFOA, but confirm the certification covers the specific contaminant.
One caution that surprises people: do not boil tap water to make it safer during pregnancy. Boiling kills microbes, but it evaporates water and concentrates metals like lead, arsenic, and mercury rather than removing them.
Why We Set a Number Per Group, Not One for Everyone
Most water resources publish a single threshold per contaminant and apply it to a grown adult and a first-trimester pregnancy alike. We don't. CheckYourTap builds safe levels per population group, pregnancy, newborns, infants, older adults, even dogs and cats, because a body still assembling its detox pathways cannot handle the concentration a healthy adult clears without trouble. We anchor each number to what protects that body, not to what utilities can cheaply achieve. That is a slower, more deliberate way to build a database, and we'd argue it's the only honest one. The live personalized report covers Connecticut today, and we're expanding state by state.
The Bottom Line for Expecting Parents
"Meets federal standards" answers a legal question, not a biological one. All five of these contaminants cross the placenta, and for lead and PFOA no exposure is considered safe. Arsenic's protective level is 2,500 times below the law, chromium-6's OEHHA goal is 0.02 ppb against no federal limit at all, and mercury deserves a stricter pregnancy target than the adult guideline. Test the water you drink and cook with, and if any of these show up, an NSF/ANSI 58 reverse-osmosis system closes the gap between "legal" and "safe for your baby" in a single step.
This article is for general information and is not medical advice. Talk to your prenatal provider about your specific water source, your test results, and any health concerns.
Keep Reading
- Is Tap Water Safe During Pregnancy? The Gap Between Legal and Safe
- Arsenic in Well Water While Pregnant: Why 10 ppb Is 2,500x Too High
- Chromium-6 and Pregnancy: The Real Safe Level
- Lead in Drinking Water During Pregnancy: Why There's No Safe Level
- Lead: sources, health effects, and safe levels
- Arsenic: sources, health effects, and safe levels
Sources: California OEHHA Public Health Goals (arsenic 0.004 ppb, 2004; hexavalent chromium 0.02 ppb, 2011; mercury 1.2 ppb); EPA PFAS National Primary Drinking Water Regulation (PFOA MCL 4.0 ppt, MCLG 0, 2024); EPA Chromium in Drinking Water (total chromium 100 ppb); EPA Lead and Copper Rule Improvements (lead action level); EPA National Primary Drinking Water Regulations (arsenic and mercury MCLs); National Research Council, Arsenic in Drinking Water: 2001 Update; EWG Tap Water Database (health guidelines); EFSA CONTAM Panel, 2020 (PFAS and fetal development); ATSDR Toxicological Profile for Arsenic, 2007; ATSDR Toxicological Profile for Chromium, 2012; ATSDR Toxicological Profile for Perfluoroalkyls, 2020; Concha et al., 1998, placental arsenic transfer; Rahman et al., 2009, arsenic and birth outcomes; Bellinger, 2005, lead and pregnancy.