Thames Valley Air Ambulance has urged businesses to donate protective kit because staff are "lacking the most suitable equipment" to use during the coronavirus pandemic.
The charity is not included in the official personal protective equipment (PPE) distribution system.
But it said a "major shortage" meant its staff could be left without PPE.
It said it was "inevitable" they would come into contact with coronavirus patients.
The charity, which is based at RAF Benson in Oxfordshire, said it was "keen not to add pressure to official stocks if possible".
It currently serves Oxfordshire, Berkshire and Buckinghamshire and has urged businesses to donate items including surgical masks and anti-bacterial wipes.
Can you help in the fight against #Covid19? We are looking for businesses able to donate the following #PPE; Disposable FFP3 Respirator Masks; Fluid-resistant (Type IIR) Surgical Masks. Please email email@example.com or visit https://t.co/N1RGhYGMly for more information. pic.twitter.com/bEvM7FFBVD— Thames Valley Air Ambulance (@TVAirAmb) March 31, 2020
Martin Bowdler, Thames Valley Air Ambulance's head of clinical service, said: "People in our communities still needs our hospital-level critical care as, despite the pandemic, there are of course still those who will face life-threatening illnesses and injuries.
"That's why it's vital that our crew are protected from Covid-19 so they can continue delivering advanced medical care without spreading or contracting the virus.
"We are appealing to businesses to donate stocks of PPE to keep our crews safe while working on the frontline by ensuring that they have the right equipment to give patients the best possible chance of survival and recovery."