NHS England's chief executive says extended industrial action is "clearly having an impact".
Patients are at risk as hundreds of thousands of helpline calls go unanswered, BBC research finds.
Dafydd Williams has lived in Wales for 20 years but still has to travel to England for check-ups.
It's the first day of action by the Unite union and follows two days of GMB walkouts.
A spokesperson for the Department of Health and Social Care in England said: "We have introduced tough regulations to deter the appeal of vaping...
You are never too young to start improving your brain health, an Alzheimer's charity says.
One out of every four hospitals and community services will be affected by the two 12-hour walkouts.
Two days of action - on 6 and 7 February - are announced in England and Wales, in the biggest walkout yet.
A Trust knew about the exposure for more than a year before informing staff, documents show.
The RCN wants progress in negotiations by the end of January or says it will ballot more members.