They can also be a magnet for anti-social behaviour and can have an adverse effect on the environment and can lead to a spiral of decline in certain communities.
Maent hefyd yn gallu denu ymddygiad gwrthgymdeithasol a chael effaith andwyol ar yr amgylchedd, ac yn gallu arwain at drobwll o ddirywiad mewn rhai cymunedau.