Ambulance workers are on strike after claiming a change in shift patterns will put patients' safety at risk.
Members of the Unite union in Yorkshire walked out for 24 hours at midnight, and will do so again for four hours from 3pm on Monday.
They claim some paramedics could work for 10 hours without a meal break under new longer shift patterns.
A health chief has attacked the strikes as "reckless", but the union said the changes will impact on patient safety and is calling for a 30-minute meal break every six hours.
The Yorkshire Ambulance Service NHS Trust said the industrial action would affect services, involving around 8% of its staff.
The trust, which is to stop recognising Unite for collective bargaining, receives an average of 2,100 emergency calls a day.