Ambulance drivers will, on Friday, carry only emergency work after the Union Haddiema Maghqudin issued a one-day directive.
Workers are complaining that the air quality in the garage is poor because of an inadequate ventilation system.
The health authorities were notified of the dispute yesterday afternoon but a spokesman said the union failed to specify what action will be taken.
The union sounded its battle cry last week. UHM health section secretary Joseph Bonello had then said the union was considering directing drivers to stick to emergencies only for a day and not to collect patients who have outpatient appointments.
Contacted yesterday, Mr Bonello said up to 10 ambulances carried out a number of trips to ferry outpatients. In fact, he said, there was rarely an ambulance in the garage before 2.30 p.m.
A Health Division spokesman said the division would react according to the action taken by the union.