Despite recent reports of the spate of homes needing P decontamination in New Zealand, experts say the risks of toxicity from living in a house where the Class A drug has been smoked are minimal.
Housing New Zealand (HNZ) has released figures showing the number of state houses testing positive for methamphetamine contamination is on the rise. Chief operating officer Paul Commons said about 400 HNZ properties around the country were uninhabitable as a result of P contamination and were in various stages of testing and remediation.
However, the growing alarm at houses where drug users have lived may be misdirected.
"People dwelling in a house where previous tenants had smoked methamphetamine, and there is some evidence of low concentrations on surfaces, have minimal risks of toxicity," says toxicologist Dr Leo Schep of the National Poisons Centre.
"The risks would be similar for people who live in a house that had previous dwellers who smoked cigarettes or marijuana. They will have exposure to these drugs, but the concentrations will not be sufficiently high enough to cause either psychoactive or toxic effects to people who may have had inadvertent, and brief, dermal contact with these surfaces."
The confusion may be between houses where methamphetamine has only been consumed and houses where it has been manufactured, where the risks are far greater.
"When addressing problems associated with contaminated houses, we are dealing with two separate issues: a house where someone smoked methamphetamine, or a house that was used to manufacture methamphetamine," says Dr Schep.
"People living in a laboratory environment risk suffering adverse cardiovascular, respiratory and dermal effects following the exposure to organic solvents, acids, alkalis and other chemicals."
Dr Nick Kim, senior lecturer in applied environmental chemistry at Massey University, says the main ways people living in a P-contaminated house would be exposed to harmful toxins are inhalation, absorption through the skin or oral ingestion.
"Skin absorption is almost always a minor and secondary exposure pathway, because the skin -- unlike the lungs -- is designed to operate as a barrier. However, skin absorption is the main exposure pathway for methamphetamine residues. Breathing is the main exposure pathway for solvents [used in P manufacture]," Dr Kim tells the Science Media Centre.
"Once solvents are absorbed by soft furnishings and wall boards they gradually will re-volatilise back out into the air and be inhaled. Although some of these are solvents that we are occasionally exposed to, for example nail polish remover, we generally don't want children or adults to be breathing them for extended periods, even at low concentrations."
Dr Kim says decontaminating a house in which P has been consumed -- rather than manufactured -- can be fairly simple.
"A key priority in site remediation should be to ensure that residual concentrations of solvents in the air are below relevant guideline levels or non-detectable. This can start with removing and replacing soft furnishings and keeping doors and windows open for an extended period, but in severe cases might require removal and replacement of the wallboards or perhaps material inside the ceiling cavity."
However, Dr Kim says it is impossible to establish what a safe level of methamphetamine residues on surfaces would be.
"I personally don't think it is possible to accurately specify that, nor will it ever be, for several reasons; including being unable to predict the frequency of hand washing or how much wall someone's bare skin will come into contact with," says Dr Kim.
"For these reasons my own view would be that calls to specify a higher 'no longer safe' level of methamphetamine are calls for something that is technically unachievable. However, I am reasonably confident that if the guidelines are met, then any exposure would be negligible."