Where the 'credit card of plastic a week' figure comes from

The claim that a person eats "a credit card's worth of plastic every week" is the most repeated sentence in the microplastics conversation. It traces to a single figure — about 5 grams a week — and that figure is the upper end of a range so wide it spans a factor of fifty. Read against a second study that modelled the mass mechanistically, the realistic number is measured in micrograms, not grams.

The 5-gram figure was popularised by a 2019 WWF-commissioned report and later underpinned by a peer-reviewed analysis published in 2020. That analysis pooled data from 59 publications on microplastic counts in food, water and air, converted particle counts into an estimated mass, and concluded that humans "may ingest 0.1–5 g of microplastics weekly through various exposure pathways" [s1]. The authors were explicit that this was "the first attempt to transform microplastic counts into a mass value relevant to human toxicology" — a first step, not a settled number [s1].

The number that travelled was the ceiling

The interval reported was 0.1 to 5 grams per week [s1]. The bottom of that range is fifty times smaller than the top. Public retellings kept the 5-gram end, because 5 grams is roughly the mass of a credit card and the image is vivid. The 0.1-gram end, and the fifty-fold uncertainty between them, did not survive the translation into a headline.

Converting particle counts into mass is where the uncertainty lives. A mass estimate depends on assumptions about each particle's size, shape and density, and small changes in those assumptions move the total by orders of magnitude. Counting a particle is one measurement; assigning it a mass is several more, each with its own error.

A model built from the other direction

A separate 2021 study took a mechanistic approach. Instead of scaling counts up to a mass, it built a probabilistic lifetime-exposure model accounting for intake through eight food types plus inhalation, along with intestinal absorption and excretion [s2]. It reported median microplastic intake rates of 553 particles per person per day for children and 883 particles per person per day for adults [s2]. In mass terms, those medians correspond to 184 nanograms per day for children and 583 nanograms per day for adults [s2].

583 nanograms a day is a figure in the micrograms-per-week range — roughly a millionth of the gram-scale that the credit-card image implies. The same model estimated that microplastics irreversibly accumulating in body tissue reach on the order of 8.32 × 10³ particles, or 6.4 nanograms, by age 18 in children, and up to 5.01 × 10⁴ particles, or 40.7 nanograms, by age 70 in adults — each within wide confidence intervals [s2]. These are small masses, accumulated over a lifetime, not a weekly gram.

The two studies are not measuring exactly the same thing, and the gap between them is partly a gap between an upper-bound scaling estimate and a central mechanistic one. But that is the point: the "credit card a week" line takes the least certain, highest number from a range and drops both the uncertainty and the lower bound.

Why the mass matters less than it sounds

There is a further wrinkle the credit-card framing obscures. The 2021 model also examined the chemicals that ride along with ingested plastic and concluded that "the contribution of microplastics to total chemical intake is small" relative to other exposure routes [s2]. In other words, even the plastic-associated chemicals people worry about mostly reach the body by paths other than swallowed particles.

None of this says microplastic exposure is zero or harmless. Particles are measurable in food, water and air, and both studies exist because the exposure is real enough to quantify [s1][s2]. The question is whether the quantities are ones the body responds to — and that is a separate question about dose and biological effect, not about how many grams sound alarming.

What the figure is good for

The 5-gram number is best understood as what it was designed to be: an early, deliberately conservative attempt to put any mass value on human microplastic intake, useful mainly as a starting point for risk assessment [s1]. Its authors framed it that way. It became a slogan because a slogan needs a single number and a familiar object, not a range and a set of assumptions.

A more defensible summary is that adults ingest on the order of hundreds of particles a day, a mass in the micrograms-per-week range on current modelling, with wide uncertainty in either direction [s2]. That sentence does not fit on a magnet. It is closer to what the evidence says. And it changes what the number is for: not to frighten, but to mark a real exposure whose health significance is still the open question — one that a gram-scale slogan makes harder to see, not easier.

This article is informational and is not medical advice.

Sources

• Estimation of the mass of microplastics ingested – A pivotal first step towards human health risk assessment — Journal of Hazardous Materials, 2020-10-06
• Lifetime Accumulation of Microplastic in Children and Adults — Environmental Science & Technology, 2021-03-16