Japan is one of the safest destinations in the world..... probably safer than where you come from. The Japanese have a reputation for being scrupulously honest, and as one traveller said to me after his first visit "It feels like the sort of place where you could leave a briefcase stuffed with cash in the middle of the road and it'd still be there a day later.".
If you're worried about radiation etc since the tsunami last March, a work colleague has family who live close to Tokyo (even closer than Yokohama) and they're eating the food and drinking the tap water. If - IF - there's any increased background radiation in Japan because of the Fukushima Daiichi incident, from what I've read, it is most likely to be relatively low and localised. Yokohama is far enough away not to be a concern. By way of example - on the day that Tokyo received radiation levels 20 times normal, the actual level was equivalent to an abdominal x-ray.
World Nomads has a stack of articles about safety in japan, try starting with this one.
Good luck and have a great time.
Phil almost 8 years ago