There are about 1,000 attacks a week on health workers
Ambulance services across the UK hold records of more than 8,500 households who have a history of violence against paramedics, the BBC has learned.
Staff called to emergencies at these addresses are advised to wait for police before entering the premises.
The Patients Association says records must be "absolutely accurate" or "patient safety may be in danger".
Unison is calling for the system to be reviewed, saying there are "serious questions" about how it works.
The union says paramedics are faced with a moral dilemma, forced to make a difficult choice between their own safety and that of their patients.
Karen Jennings, from Unison, said: "I think there are serious questions to ask about whether ambulance crews should sit outside if somebody inside is having a heart attack.
Liz Kendall of the Ambulance Service Network on the "unacceptable attacks"
"Having said that, if that household has a history of attacking people when they go in, then it doesn't do anybody any good if they were just to rush in and put themselves at risk."
There are an estimated 1,000 attacks a week on health workers and members of the emergency services.
Fear of violence has led to all paramedics in London being offered stab-proof vests to wear on duty, and increasingly, there are calls to hand them out to staff nationally.
Another way of responding to the problem has been for ambulance services to keep a record of addresses where there have been abuse, threats or attacks.
BBC Radio 5Live's Donal McIntyre programme has obtained a region-by-region breakdown of those records.
The West of England division, which covers Bristol, Swindon and Gloucester, currently has 312 addresses flagged as "at-risk". About half were reported for physical violence and 58 for sexual attacks.
In London, the number is 1,930, but the service did not specify the reasons why properties were included.
The North West has the most flagged households - 3,071 - but a spokesman pointed out that they made up just 0.10% of all residential addresses.
Andy Price, a paramedic in the West Midlands, told the programme he was assaulted when responding to a call for an arm injury.
"When we arrived on the scene it very quickly became apparent that the reason for which we had been called was perhaps erroneous," he said.
"There was a number of people there - they had all been drinking quite heavily. My colleague, while attempting to calm the situation, was punched in the face.
"I was later punched in the face myself. I required several days off work."
Another paramedic who has more than 20 years service at the North East Ambulance NHS Trust believes the problem has got worse recently.
"Even 10 years ago people wouldn't dream of verbally abusing you - now with drinking and drug problems it's much more common," he said.
The Patients Association says flaws in the system could put patients at risk.
Spokeswoman Vanessa Bourne said: "A record like this has to be absolutely accurate and up to date. If they are not, patient safety and lives may be in danger.
"There are many reasons why a patient's behaviour puts them on the register, but they may be the patients most in need of care."
The recent Emergency Workers (Obstruction) Act 2006 makes it an offence to obstruct or hinder persons who provide emergency services.
The Donal MacIntyre programme will be broadcast on BBC Radio 5 live at 7pm on Sunday 2 November, 2008 or download the programme