Nurses' strike NI: All you need to know
An estimated 15,500 nurses are taking strike action across Northern Ireland in protest over pay and staff numbers.
The figure includes about 9,000 nurses who belong to the Royal College of Nursing (RCN) union and about 6,500 who are members of Unison.
The widespread strike is unprecedented as it is the first time in the RCN's 103-year history that its members have gone on strike anywhere in the UK.
Members of other unions, including ambulance staff, are also on strike.
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When is it happening and who is involved?
RCN members began a 12-hour strike at 08:00 GMT on Wednesday, following previous industrial action by its nurses which stopped short of a strike.
Many Unison health staff members, including nurses and paramedics are joining the strike, with ambulance staff staging a 24-hour strike.
There will be 24-hour strike action by all Northern Ireland Public Service Alliance (NIPSA) members in the health services beginning at 00:01 GMT and concluding at 23:59 on Wednesday.
What services will be affected?
The Health and Social Care Board said "major challenges are expected across all health and social care services in Northern Ireland".
"Regrettably, as a result of the widespread nature of the strike, numerous appointments and treatments have been cancelled and many services across our hospitals and the community care sector have had to be stood down or reduced," the board said.
Minor Injury Units at the South Tyrone, Mid Ulster, Bangor and Ards hospitals will be closed on Wednesday.
It said all emergency departments will remain open as normal, but will be under significant pressure, it said.
Patients who are seriously ill or injured should go to the emergency department, but the HSCB says if you do not need emergency care please choose alternative services.
The Trust says some outpatient, inpatient and community care appointments have been cancelled, but any patients affected will be contacted directly. All impacted areas may be found on their website.
There will be "significant disruption" to health and social care services across the Southern Trust. For a list of affected and cancelled services and the full list of day centres please see the Southern Trust website.
South Eastern Trust
The South Eastern Trust has said it is expecting "widespread cancellations and disruption to services". Patients will be contacted if their procedure, appointment or service has been cancelled and if they have not they are advised to attend.
Any cancelled appointments will be rescheduled automatically. For a list of changes please see their website.
The Western Trust says action will have widespread impact on the provision of all its health and social care services.
Patients and service users that are directly affected have been contacted but please see the website for updates and cancelled or disrupted services.
Northern Ireland Ambulance Service Trust
The Northern Ireland Ambulance Service warned of "significant disruption" to its services throughout the 24 hour period of action.
NIAS said it has been "engaging constructively" with trade union colleagues to ensure, as far as possible, a response to the most clinically urgent calls and to cancer, renal and palliative patients.
It will prioritise calls to provide the quickest and most appropriate response to meet the clinical needs of patients.
Calls to the 999 service which are less serious in nature, potentially, face a delay in response times and NIAS asked for patience from callers in this regard.
However, it stressed if you believe you need to call 999, please do not hesitate.
The latest information on exemptions to strike action can be found on the Health and Social Care Board website.
What have the unions said?
The RCN Northern Ireland has issued a list of services that will be exempt from next Wednesday's planned strike action by health unions.
They include chemotherapy inpatient and outpatient services, units providing palliative care, psychiatric intensive care units and intensive care units.
Emergency departments will not be exempt, but life-saving cover will be provided, the RCN added.
The list has been given to its members, health and social care employers and the Department of Health.
To ensure patient safety and protect registered nurses, the RCN has said it is "vital that we make sure there are sufficient numbers of registered nurses and nursing assistants to provide essential care for their patients."
Kevin McAdam from the Unite union said the trade unions were "working hard" to ensure there was necessary staff cover.
Anne Speed from Unison said joint meetings had taken place with employers on Tuesday and that it had provided an exemption from striking for staff working in "cancer treatment and children's homes".