A private hospital in Kenya has banned masks with valves within its premises citing one way protection.

The Aga Khan University Hospital announced that masks with valves only protect the wearer but not those around them.

"The design of these masks allow inhaled air to be filtered but the exhaled air is not. The exhaled air can be expelled towards others even more than from a person not wearing a mask," the notice read in part.

The hospital said the masks are risky to other people:

The masks with valves are common in Kenya.

The World Health Organisation (WHO) does not recommend the use of masks with exhalation valves.

"These masks are intended for industrial workers to prevent dust and particles from being breathed in as the valve closes on inhale. However, the valve opens on exhale, making it easier to breathe but also allowing any virus to pass through the valve opening. This makes the mask ineffective at preventing the spread of COVID-19 or any other respiratory virus," a WHO guideline for masks reads.