The use of nano-scale particles is relatively new, so it is not certain what the long term effect of using them will be on our health or the environment.
These particles are so small that they can easily pass through our skin and into our bloodstream. They may even be small enough to enter our cells or the cells of other organisms, and the effect that these nanoparticles could have is not known.
They can also enter the environment very easily and could cause harm to plants and animals.
A good example of this is the use of nano-scale silver in clothing. The nano-silver is used to kill bacteria which cause unpleasant odours. However, the nano-scale particles can enter the water during washing, and enter the environment. Due to their anti-bacterial properties, they could cause a lot of damage to bacteria within the ecosystem over a long period of time. However, there is no evidence that use of nanoparticles over a short period of time has any negative effect.