I copy and pasted this from the internet:
The benefits and risks of using foil for smoking drugs-
Services provide aluminium foil to prevent or reverse the transition to injecting drugs,
and so prevent the infections and other harms than can result from injecting. It is
offered to encourage people into treatment and on the road to recovery from drug dependance.
Services should note – and explain to service users – that, although smoking drugs
from foil will prevent many of the infection and other hazards associated with
injecting, it carries its own risks. Smoking heroin will not prevent infection from
bacteria such as anthrax and tuberculosis if the bacteria are present in the heroin.
Smoking drugs can also cause thermal damage to the throat and lungs from the heat
of the smoke. And inhaling drugs and their impurities can be damaging in itself with,
for example, heroin causing asthma-like breathing difficulties.
Services will likely want to supply foil specifically designed for drug use. There is no
evidence of harm from using household foil but some users say it irritates their lungs.