Under new measures, some critical workers in healthcare, transport and food supply will be exempt from having to self-isolate after contact with a positive Covid case.
Staff must have had their second dose of vaccine at least two weeks previously, and will have the choice whether or not to participate.
But unions warned healthcare workers may be pressured into continuing to work, in the face of extreme demand on services across Scotland.
It comes as NHS Lanarkshire asked friends and family to help with at home social care to reduce strain on staff, and NHS Lothian called for volunteers to pick up extra shifts to cover high rates of absences due to isolation.
Unison, the largest healthcare workers’ union, warned the new measures were “inappropriate”, and could have “considerable negative impact” on staff and patients.
"We have raised our concerns with the Scottish government and it’s frustrating that these have largely been ignored,” said Unison head of health Willie Duffy.
"We do not believe these new rules to exempt critical staff from self-isolation is an appropriate or well evidenced response to address the current staffing crisis.
"This could further the spread of Covid and have considerable negative impact on staff and the patients they care for.
“We are fully aware of the pressure our health and care services are under and as always, our members are going above and beyond to do everything they can.
"But asking staff to adhere to the new rules is creating a double standard and as such UNISON Scotland cannot support these new rules.”
The Royal College of Nursing said the rules and the lack of clarity around them made nursing staff feel “unvalued”, and risked both staff and patient safety.
The new guidance was announced at midday on Friday, but guidance on how it applied to health workers was not given until 6pm, despite repeated requests from journalists and union officials.
Julie Lamberth, Chair of the RCN Scotland Board, said: “This message from government will be confusing to health and care workers who have spent over a year doing their best to keep the people they care for safe from infection.“Although self-isolation exemption rules outlined today by Scottish government are based on voluntary participation by health and care staff, there is the potential for increasing pressure from hard-pressed services on staff to return to work.
“We have not seen the evidence behind this position, including how many staff will be affected. After waiting all day for clarification, first impressions are that the guidance appears lengthy and complex… it is difficult for staff to make an informed decision as our members head into weekend working.
"This doesn’t just make nursing staff feel unvalued, the government is risking both patient and staff safety.”
Dr Lewis Morrison, chair of doctors’ union BMA Scotland, welcomed the announcement, but warned: “A return to work under this policy has to be voluntary, and there must be no inappropriate pressure on staff to return to work in these circumstances.
"Use of this policy to keep services going should only be in extremis, and could only ever be a very short-term measure.”
Hospitality and retail groups have called for workers to be included in self-isolation exemptions to relieve staff shortages.
The Scottish Hospitality Group warned businesses were being forced to closed temporarily because of self-isolation, and called for a more lenient approach.
The Scottish Retail Consortium said the change was a step forward, but that further action should be taken to include more workers if needed.
The new rules also apply to transport workers.
Scotrail will consider whether or not to implement them, a spokesperson said, while maintaining the safety of staff.
Loganair chief executive Jonathan Hinkles welcomed the move, saying it has become “increasingly challenging” to keep services running while staff have been required to self-isolate.
Scottish Labour’s Jackie Baillie called the move a “sticking plaster solution”.
“We do need a plan to ease self-isolation requirements but the Government’s sticking plaster solution raises as many questions as answers,” she said.
“The current situation of staff shortages faced by businesses and essential services is not sustainable, but changes cannot come at the expense of frontline workers’ safety or public health.”
Liberal Democrat Willie Rennie said he wanted to further study the detail of the change.
"If the safeguards are right and the risk of the spread of the virus is low then this would be a sensible step,” he said.
The Scottish Government said the guidance will apply to healthcare workers “in limited circumstances, on a case by case basis and only where absolutely necessary.”
A spokesperson said: “Staff have the right to freely choose whether or not to return to work if asked if they are willing to do so by their employers.”
Announcing the change, First Minister Nicola Sturgeon said: “We have seen significant staff shortages in a small number of organisations in recent days and we have worked with them to protect services. Applications for exemptions are being considered from today and we will consider applications as they come in.
“Clinical evidence tells us we can safely and effectively release some critical staff from self-isolation, with appropriate safeguards. However, this is a very limited change at this stage, to be applied on a case by case basis and only where absolutely necessary.
“We will not allow key services to be threatened by staff shortages but equally we must continue to protect public health.”