What Tiny Plastics Leave in Water: Heat and Time Intensify Toxic Effects in Lab Tests

A laboratory study found that water exposed to three common plastics disrupted seed germination and altered bull sperm movement, with stronger effects linked to smaller fragments, higher preparation temperatures and longer contact with water.

A plastic fragment can leave more than a visible speck in water, releasing a mixture of substances that may affect living cells. A laboratory study found that water exposed to three common plastics disrupted seed germination and altered bull sperm movement, with stronger effects linked to smaller fragments, higher preparation temperatures and longer contact with water.

The research, "Microplastics in Water: Multi-Factor Experimental Assessment of Aqueous Leachate Toxicity and a Framework for Human Health Risk Screening," published in Water, examined these responses through several biological tests. Researchers from Kazakhstan and Spain developed a screening framework to identify concerning results that an overall average could conceal. The findings describe potential hazards under laboratory conditions, without establishing that everyday exposure causes illness or infertility in people.

Three familiar plastics tested beyond the visible fragments

The team studied polypropylene, or PP, and polyethylene, or PE, taken from water-supply pipes, alongside polyethylene terephthalate, or PET, from a plastic bottle. The materials were cut and ground into five particle-size groups ranging from 0.175 to 3 millimetres, then left in distilled water for 30 days.

For wheat and lentil tests, the water was prepared at 25°C, 37°C and 60°C, with all seeds germinated at 25°C. Separate leachates prepared at 40°C were used for bull sperm and rabbit-skin tests, meaning the experiments examined different biological responses rather than a single shared exposure condition.

The resulting liquid, known as leachate, was filtered to remove larger fragments. The filter could allow particles smaller than 2.5 micrometres to pass, including possible nanoplastics, so the tests cannot separate the effects of dissolved chemicals from those of extremely small particles. The researchers assessed the mixture's biological activity without identifying and measuring its individual chemical ingredients.

Hotter preparation and smaller fragments weakened seed performance

Wheat and lentil seeds generally performed worse after exposure to leachates prepared at higher temperatures, especially 60°C. The smallest fragments, measuring 0.175–0.3 millimetres, tended to produce the strongest effects, a pattern the authors linked to their greater surface area in contact with water and the potential for increased chemical release.

Polypropylene produced the strongest overall inhibition in the plant tests, followed by PET and polyethylene. Lentils were generally more resistant than wheat at 25–37°C, although their germination performance fell sharply with leachates prepared at 60°C. Larger fragments often produced responses close to the clean-water controls at the lower preparation temperatures.

The researchers combined germination success and speed into a modified Germination Index, with lower values indicating greater inhibition. For wheat exposed to polypropylene leachates, the index averaged across particle sizes fell from 72.62% at 25°C to 51.18% at 60°C; the lowest individual scenario reported in the paper reached 38.20%. These figures describe early seed performance, without measuring harvest yields or showing equivalent effects inside the human body.

Sperm movement changed without visible skin irritation

The ranking changed in the bull sperm tests, where PET produced the greatest average deviation in movement from the control, followed closely by polypropylene. Mean motility deviations were 16.18% for PET, 15.62% for polypropylene and 8.92% for polyethylene; the largest individual change was 29.1% for PET fragments measuring 0.3 millimetres.

Contact time mattered when the results were compared with the team's earlier one-day leaching experiments. Mean motility deviation rose from 5.02% to 16.18% for PET, from 4.64% to 15.62% for polypropylene, and from 2.16% to 8.92% for polyethylene after 30-day preparation. The increases were roughly threefold to fourfold, suggesting that a short test can miss biological activity developing during prolonged contact.

Leachates associated with poorer seed performance tended to produce larger changes in sperm movement, with a moderate inverse relationship between the two responses (correlation coefficient −0.547). This pattern supports using several biological tests to investigate a mixture, because different cells and organisms can respond differently to the same plastic source.

Rabbit-skin testing produced no visible irritation after a single four-hour application, with observations made one and 24 hours after exposure. The irritation score was zero across the tested samples, a result limited to this short exposure on intact skin that cannot establish safety for repeated contact, damaged skin or ingestion. Changes in bull sperm movement provide a laboratory warning signal rather than evidence of reduced human fertility.

Why an average score can hide a stronger warning

The researchers combined the findings across 45 combinations of plastic type, particle size and leaching temperature using a weighted Total Risk Index. Scores ranged from 0.012 for coarse polyethylene under cooler conditions to 0.400 for fine polypropylene at 60°C, leaving every scenario below the framework's high-risk cutoff of 0.51.

A second method assigned each scenario the category of its strongest adverse response. Fine PET and some polypropylene scenarios received high hazard classifications under this approach, because pronounced sperm or seed effects were no longer softened by lower responses elsewhere. Polyethylene produced the lowest overall responses, and none of its scenarios crossed the framework's high-risk thresholds.

One PET scenario received a moderate combined score of 0.250 despite a 29.1% sperm-motility deviation that crossed the authors' high-category threshold. Giving zero skin irritation a place in the weighted average helped lower the total, illustrating how a concerning result can become less visible. Removing the skin component and adjusting the weights raised the strongest polypropylene score from 0.400 to 0.476, still below the high-category cutoff.

These categories belong to a proposed hazard-screening system, rather than validated measures of human health risk. The framework combines plant data from several preparation temperatures with sperm data collected only at 40°C, and it does not calculate actual exposure through drinking water. Its equations apply within the tested particle-size range and cannot reliably predict nanoscale behaviour.

The authors recommend broader testing of plastics used in water systems and containers, covering prolonged contact, warmer conditions, different fragment sizes and sensitive mammalian-cell responses. Chemical analysis, realistic exposure measurements, studies of how substances enter and leave the body, and repeated-exposure research are needed before human risks can be quantified. The study makes a practical case for assessing what plastic leaves in water and ensuring that an average score does not obscure a strong biological warning.