A lone paramedic had a substance thrown on to her face, neck and chest while answering a 999 call.
She was on her way to a patient when she was flagged down by three men who appeared to be in distress in Tottenham Hale, north London.
The 32-year-old stopped to help but they pulled bandanas over their faces before one threw liquid through her window.
She was taken to hospital after the attack on 16 July but later discharged.
A Met Police spokesman said the substance was non-corrosive but it is investigating the incident.
Since 2010, there have been more than 1,800 reports of attacks involving corrosive fluids in the capital. Last year, it was used in 458 crimes, compared to 261 in 2015, according to Met Police figures.
The paramedic, who did not want to be named, was on her way to help a man suffering from chest pains when she was targeted in the early hours.
"It was terrifying. This was so cowardly," she said.
"It is my job to help people. I was on my way to help a patient and I stopped because I am caring and I thought they needed my help.
"They have taken away my trust.
"What they've done is horrific in so many ways. It was premeditated and it delayed a patient getting treatment."
She said the attack took a paramedic off the road that night.
"And yet if one my attackers were hurt, I would still treat them because that is the job," she added.
The substance caused irritation to her face, neck and chest. The man who threw it was wearing latex gloves.
London Ambulance Service is reminding its staff of the need to be cautious when flagged down by anyone requesting help or assistance.