The "unprecedented" pressure hospital emergency departments say they are experiencing could get worse under NHS reforms, a health expert has warned.
A&E departments across Wales are facing a huge rise in demand with some hospitals treating more patients than ever before, the Welsh government says.
Ambulances are also facing delays handing over patients at hospitals.
Health academic Marcus Longley said proposed closures of some A&E units could make health hot spots worse.
Plans to reform the way services are delivered have been drawn up by health boards amid warnings from First Minister Carwyn Jones that the health service would "collapse" without reform.
Some changes could involve people having to travel further for treatment.
In an interview with BBC Wales' Good Morning Wales programme, Prof Longley warned that unless the changes and closure of some A&E units are handled properly, it could make the current situation worse.
"It could certainly increase some hot spots, there's no doubt about that, because if it's a 10 minute journey to the A&E department, in the future it will be 20 minutes," said Prof Longley, director of the Welsh Institute for Health and Social Care at the University of Glamorgan.
"That's an ambulance that's tied up for that much longer if the call was unnecessary.
"I think all of that emphasises the importance of getting the key bits of the system right.
"If all you do is change bits here and there, that doesn't really solve the problem.
"But the health service is struggling to know what to do about this."
Health boards across Wales say that demand over the past few days has been exceptionally high.
More than 450 patients were seen at the University Hospital of Wales, Cardiff, on Monday - a quarter more than expected - with almost half needing high level care.
And a new record was set at the A&E department at Morriston Hospital in Swansea on the same day according to the Welsh government - with over 300 patients turning up.
Hywel Dda Health Board said some operations at its hospitals in mid and west Wales had been postponed as a result of the high number of hospital admissions, but it had increased bed numbers to cope with the influx.
The Welsh Ambulance Service also said it was concerned at the handover delays at hospitals.
Chris Thomas, chair of the Welsh board of the Royal College of Nursing, said the usual "winter spike" in emergency cases seemed to be "bigger than ever before".
"It used to be winter pressure, now it seems to be the better part of the year actually," she added.
Roy Norris, a former chair of the Welsh Ambulance Trust who currently sits on the Montgomery Community Health Council, warned the problems would not easily go away.
He said it was "not uncommon" to have pressure when there's cold weather.
"I'm sure the planners are really worried that when this cold spell comes to an end there will be a further sharp increase in heart attacks and strokes," he added.
"So this crisis isn't going to be over in a few minutes. It's going to run, I would guess, for at least 10 to 12 days."
Darren Millar AM, the Conservatives' spokesman on health, said the current situation was a "perfect storm for a cash-starved health service already hanging on by its fingernails".
"These pressures are likely to result in longer waiting lists, delayed ambulance response times, and increased risk for patients," he said.
"We need swift assurances that these severe problems have not been caused by on-going financial chaos and looming end-of-year debt."
The Welsh government acknowledged there has been a significant increase in emergency pressures during the last week.
"This is being managed locally by senior executives at local health boards and the Welsh Ambulance Service. Officials have been monitoring the situation closely," a spokesperson said.
"The public have an important part to play at this challenging time for emergency care services by using the most appropriate healthcare service to their needs and thinking before attending the emergency department or dialling 999."
A free "Choose Well" application for smartphones has been launched to provide advice on which part of the health service people should contact when ill or injured.