This study aimed to assess the levels of heavy metals in the breast milk of women residing in the mining and agricultural areas of East Azerbaijan province in Iran. This cross-sectional study analyzed 68 lactating mothers from mining (n = 28) and agricultural (n = 40) areas of East Azerbaijan province in Iran between June 2022 and March 2023. The study used an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to measure the concentrations of heavy metals, including arsenic (As), chromium (Cr), copper (Cu), and iron (Fe). A semi-quantitative food frequency questionnaire (SQ-FFQ) was used to collect data on the dietary and sociodemographic characteristics of the mothers. Although the concentration of arsenic (As) was below the limit of detection (LOD), the mean concentrations of chromium (Cr), copper (Cu), and iron (Fe) were 1.11, 0.87, and 13.25 mg/L in agricultural areas and 0.83, 0.93, and 11.35 mg/L in mining areas, respectively. The concentrations of Cr (p <
0.002) and Fe (p = 0.019) were significantly higher in the breast milk of women residing in agricultural areas. However, the concentration of Cu was significantly higher (p = 0.085) in the breast milk of women living in mining areas. Additionally, lactation age had a significant effect on Cu levels (p = 0.015), with a negative coefficient of -0.011. The study indicates that the levels of heavy metals in breast milk can be influenced by the exposure to pesticides, fertilizers, volcanic soil, and disparities in access to post-natal care and iron supplements.