# Microplastics: evidence, pathways, and uncertainty

**Age range:** 12–18  
**Time:** 45–60 minutes  
**Materials:** paper, pens, and access to the source table at `/global-data`

## Learning objectives

Students will be able to:

1. distinguish fragments, fibres, films, beads or pellets, and foam;
2. map how a plastic product can shed and move through water, soil, and air;
3. identify a study's sample, units, analytical method, and lower size limit; and
4. explain why two reported particle counts may not be directly comparable.

## Starter — size without a microscope (5 minutes)

Write these values on the board: 5 mm, 70 µm, and 1 µm.

- 5 mm is the common upper boundary for microplastics.
- 70 µm is an approximate width of a human hair.
- 1 µm is a commonly used boundary for nanoplastics, though definitions vary.

Ask: If one instrument sees particles down to 100 µm and another sees down to 1 µm, should their totals be compared as if they counted the same thing? Why?

## Activity 1 — classify particle forms (10 minutes)

Give each group the labels **fragment**, **fibre**, **film**, **bead/pellet**, and **foam**. Students sketch each shape and add one plausible source. Emphasize that shape alone does not chemically confirm that a suspected particle is plastic.

## Activity 2 — garment pathway (15 minutes)

Students draw a pathway beginning with a polyester garment:

manufacture → wear → washing or drying → filter/treatment → water, sludge/soil, or air

At every arrow, ask students to mark:

- what physical process moves or releases fibres;
- where capture could occur; and
- what local information would be needed to know how important the pathway is.

## Activity 3 — compare two studies (15 minutes)

Use two rows from `/global-data`. Students record:

| Question | Study A | Study B |
|---|---|---|
| What was sampled? | | |
| Where and when? | | |
| Particles per litre | | |
| Lower size limit | | |
| Identification method | | |
| Mean, median, or range? | | |

Then answer: Which differences could change the reported count? Can either row support a country ranking?

## Evidence-strength discussion (5 minutes)

Classify each statement:

- **Well established:** Microplastics occur in drinking water.
- **Emerging:** A particular exposure pathway is important across typical populations.
- **Still uncertain:** A reported particle count predicts an individual's disease risk.

Ask students what new evidence could move an emerging or uncertain claim.

## Exit ticket

In three sentences:

1. name one source of microplastics;
2. name one measurement detail required to interpret a particle count; and
3. explain one thing a detection study cannot establish on its own.

## Teacher guardrails

- Do not ask students to identify found particles as plastic by sight.
- Keep particle count, polymer mass, exposure, body burden, and health risk separate.
- Treat missing local measurements as missing evidence—not proof of no pollution.
- Use the site's `/resources` page for authoritative reviews and primary-study links.