"We have to hope (the ash) stays at a high altitude level so it can wash away if it rains," Tom Bellander at the Karolinska Institutet in Stockholm told news agency TT.
"If it comes down it could be a problem," he said, pointing to irritation to the eyes, nose and throat, and saying the risk could be deadly for patients with severe respiratory diseases.
But what seemed to concern health authorities in Norway and Sweden the most was that the closure of airspace meant most ambulance flights - used to evacuate sick patients in remote areas - were halted.
Scandinavian Air Ambulance, which operates ambulance helicopters in Lycksele, Östersund, Uppsala, Stockholm and Visby, announced on Friday morning that only Lycksele was back in service.
Norway's health ministry said its nine ambulance planes and 12 medical helicopters would remain grounded, and that authorities were collaborating with those in Sweden and Finland to find the fastest ground evacuation routes.
"We will do whatever we can to minimize the consequences of the grounding
of ambulance flights," Norwegian Health Minister Anne-Grete Strøm-Erichsen said in a statement.
In Sweden, the national board of health and welfare said local authorities could decide to use medical helicopters in very grave situations.
"They can decide on a case to case basis," Klas Öberg of the board's crisis preparedness unit told TT.
Local authorities in Värmland, in western Sweden, said ambulance planes would be replaced with regular ambulances, while a local hospital spokesman urged those sick with respiratory diseases to stay inside "in the case of an eventual ashfall over the country".
In Iceland, where the volcanic chaos originated, authorities called on residents living east of the site of the blast to stay inside and to wear a mask if they needed to leave their homes, Sweden's ambassador to Iceland Anders Ljunggren told Swedish public radio.
Iceland's second volcano eruption in less than a month began under the Eyjafjallajökull glacier in the south of the country at around 3am on Wednesday, spewing a cloud of ash that prompted the closure of airspace across much of Northern Europe.
Norwegian and Swedish health authorities and experts urged caution Thursday as a volcano ash cloud blew through the region, but said health risks were mostly related to the grounding of ambulance planes.
The Norwegian health minister also said Norway was preparing for possible illness brought on by the ash, but Kjetil Toerseth of the Norwegian institute for air research said health risks were minor.
"What we see now at Norwegian stations is elevated methane and elevated CO2, which can be attributed to this eruption," he told AFP.
However, when emissions of sulphur dioxide, a gas harmful to plants and trees, eventually reaches the Scandinavian peninsula the levels will be too low to pose a health risk, he said.
"When the ash and SO2 reach the Norwegian coast, I expect the ash will fall down and you will see particles in the air, (which will likely cause) soiling of laundry hanging out or a coating on a freshly-washed car," he said.