Occurrence of Microplastics in Different Brands of Bottled Water and an Assessment of Human Exposure in Detroit, Michigan

A growing number of reports document microplastic (MP) contamination in bottled water, and because bottled water is consumed directly, it represents a plausible route of human exposure; however, published concentrations differ substantially across studies as a result of non-uniform detection approaches. Here we measured MP levels across five commercial bottled-water brands, plus an ultrapure water procedural blank, all purchased in Detroit, Michigan. After vacuum filtration, samples were stained with Nile Red and imaged with fluorescence microscopy; a custom ImageJ macro then recorded particle number, cumulative particle area, and average fluorescence intensity per sample, and each brand was compared statistically to the blank using t-tests. Every sample contained detectable MPs, with brand-level mean concentrations spanning 360–634 particles/L (180–317 particles per 500 mL filter); when brands were pooled, this exceeded the blank significantly (p = 0.0001), and total particle area was likewise significantly elevated relative to the blank for each individual brand (p < 0.05). Average fluorescence intensity reliably distinguished bottled-water samples from the blank, supporting the method’s ability to separate genuine particles from background noise. Together, these results establish bottled water as a quantifiable source of MP exposure that varies by brand. Estimated Daily Intake values spanned 2.2–3.9 particles/kg/day in adults and 7.2–12.7 particles/kg/day in children, pointing to bottled water as a persistent exposure route and underscoring the need for standardized measurement protocols and greater regulatory scrutiny of packaging as a contamination source.