A group of schoolgirls claims to have made a scientific breakthrough that shows wifi signals could damage your health - by experimenting with cress. The 15-year-olds set out to test whether mobile phone signals could be harmful. They say the result could affect millions of people around the world. An experiment in Denmark claims to show that Wi-Fi signals are powerful enough to kill cress seeds after just 12 days of exposure. Pupil Lea Nielsen said: 'We all thought we experienced concentration problems in school if we slept with our mobile phones at the bedside, and sometimes we also found it difficult sleeping.' However, because they were not able to monitor their brain activity at their school in Denmark, they chose to monitor plants near wireless routers, which emit similar radio waves to mobile phones. When the girls grew trays of garden cress next to wifi routers, they found that most of the seedlings died. In the experiment, they placed six trays in a room without any equipment and another six trays in a room next to two routers. Over 12 days many of the seedlings in the wifi room turned brown and died, whereas those in the others room thrived. Kim Horsevad, the students' biology teacher at Hjallerup School, said: 'This has sparked quite a lively debate in Denmark regarding the potential adverse health effects from mobile phones and wifi equipment.' The results will bolster the findings of researchers in Holland, who found that trees exposed to wireless radio signals suffered from damaged bark and dying leaves.