Belgium's government has warned citizens not to use the free cloth masks available in the country, just ten months after promising them to all Belgians during the pandemic.
Authorities say the masks contain minuscule traces of silver and a potentially dangerous chemical compound, titanium dioxide.
Initial studies show the chemicals could affect the human respiratory system when inhaled deeply, and more studies are being carried out.
Health Minister Frank Vandenbroucke told parliament on Thursday that "to be on the safe side while the studies continue, it is better to put [the masks] to one side."
Last year, the government ordered 18 million masks from Luxembourg-based company Avrox and began distributing them for free in June through pharmacists.
But a study of fibres in the masks by Sciensano, the Belgian institute for public health, led the authorities to advise against wearing the masks as a precautionary measure.
But the institute has reiterated that they will require more research to establish if there are risks to mask users.
"The current results do not make it possible to estimate whether these nanoparticles are actually released from the masks and to what extent users are exposed to them," the study said.
"A product is not necessarily harmful if it contains certain substances. It all depends on the actual exposure and the effects of these substances."
Sciensano says they are working closely with researchers in the Netherlands and Germany to further examine the masks.
Belgium's minority government initially had limited powers to deal with the pandemic when the first cases of the coronavirus were recorded in the country last Spring.
Belgium has one of the highest per-capita death rates in the world and has registered a total of 22,000 COVID-19 fatalities.
On Friday, health authorities warned that the number of confirmed infections is rising, likely due in part to the fast-spreading variant first found in the United Kingdom.
Officials appear set to prolong restrictions that have been in place almost permanently since the start of November, including night-time curfews, the closure of certain shops, and a ban on non-essential travel