Not sure about the strength, but I'm lead to believe it does burn slightly - but the word is more hurt rather than burn! I colour my hair with normal buy-in-a-box hair colour and that burns when its left to set. One thing that helps is to put a line of vaseline around your hairline and that works like a barrier - remember to do the bits around your ears and the back of your neck - easy to miss if you have long hair.
it can burn! when i was young my mum offered to do my roots as i was a natural blonde with some winter roots. we used a packet thing. i ended up with 3rd degree burns! had weird hair (half orange) for a month, waited for it to heal b4 they could fix it which cost �70 and took 6 hours! going blonde is best done by a professional!