Tens of thousands of UK council staff are set to be redeployed in "critical" roles during the coronavirus crisis.
Last week, Prime Minister Boris Johnson ordered the closure of hundreds of local authority-run facilities from gyms to historic bath houses.
Council bosses are now carrying out skills surveys to see whether they can move those employees into social care, crematoriums and waste collection.
Unions said staff should not be moved into jobs "willy-nilly".
GMB's national officer Karen Leonard said: "Any role changes should be a reasonable alternative that matches the skill set and knowledge as closely as possible.
"Everyone wants to help, but safety has to be a priority. Let's apply some common sense."
In Bath, Somerset, employees of the city's famous Roman baths, museums and galleries are among those set to be redeployed in "critical roles".
In Tameside, Greater Manchester, grounds maintenance staff are now working in waste and recycling, while library staff are working in the contact centre to deal with the increase in calls.
The council is working out alternative ways to offer services, including stepping up its home library service and offering webcast funerals.
Unison, which represents 750,000 council staff across the UK said it had particular concern over people moving into refuse collection and adult social care.
"There are certain areas in adult social care that have particularly high-risk, vulnerable patients," said its head of local government and education, John Richards.
"You may have people who have similar caring skillsets in that field. But you wouldn't want someone who has no knowledge or understanding of those conditions."
He added that any staff being asked to take on a more skilled job during the pandemic should be paid accordingly.
The main public sector unions are urging councils not to pressurise workers into taking on new roles and to carry out strict risk assessments for those redeployed.
National officer for Unite, Jim Kennedy, said workers should also receive thorough training and the correct Personal Protection Equipment (PPE).
Skills audits are now being undertaken at most councils in the UK, many of which are expecting to have to cope with widespread absences during the pandemic.
However, the Local Government Association (LGA) said authorities were rising to the challenge.
A spokesman said: "Council staff are stepping up to this unprecedented challenge and are having to be flexible to ensure that vital local services can be maintained.
"They continue to do an amazing job in hugely challenging circumstances."
Reporting team: BBC Shared Data Unit and the Local Democracy Reporting Service
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