How to read a toxicology report

Knowing how to read a toxicology report — multiply parts per billion by the serving size in grams, and divide by 1,000 to find micrograms per serving — is one of the most empowering skills you can develop as a health-conscious consumer. If you've been following our Toxicology in Context series, you already understand that detection doesn't equal dangerOpens in a new tab. Dose, exposure, and bioavailability are what matter.

If "any amount" of a heavy metal was dangerous for you, you couldn't eat carrots, spinach, apples, or seaweed — since virtually all plants naturally absorb trace elements from soil and water. The issue isn't whether you can detect a metal in your food but whether there's enough of it to affect your health.

So how do you figure that out? By doing the math.

What if you could look at any lab report and understand — mathematically and contextually — what it means for your health? Not guess, not worry — actually know.

The math is simpler than you think, and once you see how to convert parts per billion into real serving sizes, those big numbers will start making a lot more sense.

Let's walk through it together.

What Does Parts Per Billion Mean?

Parts per billion (ppb) is a measurement of concentration. It’s a way to describe how much of one thing is present in a larger amount. In solid foods, ppb is the number of micrograms per kilogram. (Don’t worry, we’ll break this down further.)

The number itself — billion — carries psychological weight. Our brains evolved to assess threats in our immediate environment, not to intuitively or automatically grasp quantities at this kind of scale.

So what does one part per billion represent? 

Consider an Olympic-sized swimming pool. One ppb is equivalent to a single drop of water in that pool.

Said another way, a single ppb is incredibly small. When a headline or report states "27 parts per billion of lead," that means 27 micrograms of lead in every kilogram (2.2 pounds) of food. Picture those 27 grains of sand distributed throughout 2.2 pounds. That's the concentration we're talking about.

Following so far? Let’s double tap on defining a kilogram.

A kilogram (2.2 pounds) is about the weight of a small pineapple — far more than you'd usually eat in a single serving. So that 27 ppb measurement isn’t what’s found in your cup or on your plate; it represents what you’d consume if you ate roughly 7-33 servings of most foods.

Lab reports list concentrations per kilogram for scientific accuracy, but news stories rarely translate that into actual serving sizes. The typical person doesn't convert kilogram measurements to actual serving sizes but you can once you know how.

Let’s put those conversions into action with some real world examples.

How to Convert ppb to Serving Sizes (in 3 Steps)

There’s a simple 3-step formula to convert ppb into the actual amount in a single serving:

1. Find the ppb number

2. Multiply it by the serving size in grams

3. Divide by 1,000 to find out the micrograms per serving

That’s it. Now you know what’s in a portion.

For example, Girl Scout cookies made headlinesOpens in a new tab not long ago over claims about heavy metals — lead, arsenic, cadmium, aluminum and mercury. The claims originated from a reportOpens in a new tab on a website that promises to reveal the “hidden dangers” in our food.

The report labeled Peanut Butter Patties as one of the “most contaminated” cookies, citing 42.5 ppb of lead. That sounds like a lot, but that’s per kilogram (2.2 pounds) and there isn’t even a full kilogram of cookies in one box.

Let’s calculate what is in a single serving of Peanut Butter Patties:

Step 1: Find the parts per billion number. The amount given in the report is 42.5 ppb.

Step 2: Multiply the ppb number by the serving size in grams. The nutrition label shows a serving is two cookies (25 grams).

So 42.5 x 25 = 1,062.5

Step 3: Divide by 1,000 to get micrograms per serving

1,062.5 ÷ 1,000 = 1.06 micrograms per serving. 

Now remember how infinitesimal one microgram is — comparable to a grain of sand in a mini fridge. That’s roughly what you’d consume (not absorbOpens in a new tab) if you ate two Peanut Butter Patties. 

Next step: Let’s compare this to what’s naturally present in other foods.

How Much Lead Is Natural in Vegetables?

A 2018 peer-reviewed studyOpens in a new tab measured heavy metal levels in the five most consumed vegetables in the U.S: potatoes, carrots, onions, lettuce, and tomatoes. 

The researchers collected both organic and conventional samples of each one from four grocery stores in a Florida college town.

Next, they tested the samples for nine metals: arsenic, cadmium, lead, chromium, barium, cobalt, nickel, copper, and zinc. 

To keep things simple, we’re just going to focus on what they found for lead since that’s what we looked at in the Peanut Butter Patties. 

Here are the results for average ppb of lead in the five vegetables Americans eat the most: