According to the analysis, which summarises the latest knowledge on microplastics in drinking water, microplastics larger than 150 micrometres are not likely to be absorbed in the human body and uptake of smaller particles is expected to be limited.
Absorption and distribution of very small microplastic particles including in the nano size range may, however, be higher, although the data is extremely limited.
“We urgently need to know more about the health impact of microplastics because they are everywhere – including in our drinking water,” said Dr Maria Neira, Department of Public Health, Environment and Social Determinants of Health director at WHO.
“Based on the limited information we have, microplastics in drinking water don’t appear to pose a health risk at current levels. But we need to find out more. We also need to stop the rise in plastic pollution worldwide.”
One of the report's authors, Professor Peter Jarvis of Cranfield University, told Sky News that UK tap water may contain between zero and 10 microplastic pieces in every litre while bottled water can contain a few hundred.
"Where there is opportunity for water to interact with plastic material there is opportunity for plastic to go into the water source," Jarvis said.
"There are higher risks of exposure to plastics from bottled water than tap water. The evidence points to the cap itself as the main contributor to plastics in the water."
WHO said further research is needed to obtain a more accurate assessment of exposure to microplastics and their potential impacts on human health. These include developing standard methods for measuring microplastic particles in water; more studies on the sources and occurrence of microplastics in fresh water; and the efficacy of different treatment processes.
It recommends that drinking water suppliers and regulators prioritise removing microbial pathogens and chemicals that are known risks to human health, such as those causing deadly diarrhoeal diseases.
This has a double advantage: wastewater and drinking water treatment systems that treat faecal content and chemicals are also effective in removing microplastics.
Wastewater treatment can remove more than 90 per cent of microplastics from wastewater, with the highest removal coming from tertiary treatment such as filtration. Conventional drinking water treatment can remove particles smaller than a micrometre.
A significant proportion of the global population currently does not benefit from adequate water and sewage treatment. By addressing the problem of human exposure to faecally contaminated water, communities can simultaneously address the concern related to microplastics.