* Experts say small additional risk of contracting some cancers
* Girls at greater risk of thyroid cancer - report
* Breast cancer, leukaemia risk raised
* WHO urges further monitoring, health-care follow up
By Stephanie Nebehay
GENEVA, Feb 28 (Reuters) - People in the area worst affected by Japan's Fukushima nuclear accident two years ago have a slightly higher risk of developing certain cancers, the World Health Organisation said on Thursday.
A magnitude 9 earthquake and tsunami on March 11, 2011, killed nearly 19,000 people and devastated the Fukushima Daiichi nuclear plant, spewing radiation and forcing about 160,000 people to flee their homes.
It was the worst nuclear accident since a reactor exploded at the Chernobyl power plant in Ukraine in 1986.
"A breakdown of data, based on age, gender and proximity to the plant, does show a higher cancer risk for those located in the most contaminated parts," Dr. Maria Neira, WHO director for public health and environment, said of the Fukushima report.
The United Nations agency said for the general population in Japan the predicted health risks were low. But it was not able to say how many people were exposed in the area where the highest amount of radioactive material was released.
In the most contaminated area, the WHO estimated that there was a 70 percent higher risk of females exposed as infants developing thyroid cancer over their lifetime. The thyroid is the most exposed organ as radioactive iodine concentrates there and children are deemed especially vulnerable.
But experts said the overall risk was small. The radiation exposure means about 1.25 out of every 100 girls in the area could develop thyroid cancer over their lifetime, instead of the natural rate of about 0.75 percent.
"Due to the low baseline rates of thyroid cancer, even a large relative increase represents a small absolute increase in risks," the WHO said.
Thyroid cancer takes a minimum of three years to develop, according to WHO report co-author Dr. Roy Shore of the Radiation Effects Research Foundation in Japan.
"By the time people reach adulthood, susceptibility to radiation-associated thyroid cancer goes way down," he told reporters, speaking by telephone from Hiroshima.
The WHO report estimated that in the most contaminated area there was a 7 percent higher risk of leukaemia in males exposed as infants, and a 6 percent higher risk of breast cancer in females exposed as infants. Overall, girls had a 4 percent increased risk of developing solid cancers.
"These percentages represent estimated relative increases over the baseline rates and are not absolute risks for developing such cancers," the WHO said in a statement.
One-third of emergency workers were estimated to have increased cancer risks, it said.
"The risk among emergency workers would be increased for thyroid cancer particularly, and some circulatory disorders," Neira told a news briefing.
But there was no discernible increase in health risks expected outside Japan, the WHO said in a 200-page report which was based on an assessment by international experts.
"In the rest of Fukushima prefecture and in particular neighbouring countries and the rest of the world, the estimated increased cancer risk is negligible. It's within the variation of normal background rates," said Angelika Tritscher, acting director of WHO's department of food safety.
Jim Smith, Professor of Environmental Science at the University of Portsmouth in England, said: "Apart from emergency workers, the most affected people were those who remained in some highly contaminated towns and villages to the northwest of the power station for up to four months before evacuation.
"The report found that these people received a lifetime radiation dose of up to 50 milli-Sieverts (MSV) and therefore have a significant, but relatively small, additional risk of contracting cancer in later life."
Smith said the average British person receives more than 150 MSV during their lifetime from background radiation.
Neira said: "The WHO report underlines the need for long-term health monitoring of those who are at high risk, along with the provision of necessary medical follow-up and support services."
Fukushima operator Tokyo Electric Power Co (Tepco) earlier this month received approval to tap the Japanese government for 697 billion yen ($7.5 billion) to compensate those harmed by the disaster, taking the total fund to 3.24 trillion yen.
"The report was not looking at mental health or psychological impact of three major events - the earthquake, tsunami and accident at the nuclear power plant. But we know for sure that the psychological consequences and their impact will be very high," Neira said.