Response to the letters re: “The impact of New Zealand’s 2018 methamphetamine policy on residential contamination”

Dear Editor,

We thank you for providing the letters, and also sincerely thank the individuals who took the time to raise important points and offer constructive criticism of our paper.^1–4 We believe this provides us with a valuable opportunity to clarify the issues raised and explain our perspective. We trust that, after considering both the concerns raised and our responses, readers will be able to form their own assessment of the issues discussed.

Response to letter A²

Causal relationships and health impacts

We respectfully clarify that our article does not claim a causal relationship between the 2018 policy change and methamphetamine contamination. Our findings indicate that the 2018 policy change was associated with an increase in reported methamphetamine contamination in New Zealand. We also acknowledge that our study did not collect individual-level health outcome data.

Our discussion of potential health effects was based on existing literature reporting exposure and adverse health outcomes associated with environmental or passive methamphetamine exposure. For example, in the United States of America (USA), Thresher et al. (2009)⁵ used health data and reported symptoms, including nausea/vomiting, respiratory problems and eye irritation, following passive methamphetamine exposure. Grant et al. (2010)⁶ reported methamphetamine-positive test results among children after they were removed from environments where methamphetamine was being manufactured. Horton et al. (2011)⁷ also reported injuries, including injuries among children, associated with methamphetamine-related events (readers may refer to the reference list for full articles).

Nevertheless, we agree that evidence on passive methamphetamine exposure remains limited, particularly in the New Zealand context. We suggest that this represents an important research gap that warrants further investigation, as also noted in the final sentences on page 84 of our article.¹

Citation issues

We thank the authors for highlighting the citation issues. We acknowledge that some of the relevant Environmental Science and Research (ESR) (now the New Zealand Institute for Public Health and Forensic Science [PHFS]) reports were not discussed directly in our article. The more recent ESR reports were overlooked in our literature review, partly because they were published as technical reports rather than in peer-reviewed journals. Nevertheless, we agree that these reports are relevant and should be acknowledged. We also clarify that our study did not investigate the health effects of methamphetamine contamination. Our primary research question was whether the 2018 policy change was associated with an increase or decrease in methamphetamine contamination levels in residential properties. Therefore, our findings should not be interpreted as an assessment of the health risk associated with the 15μg/100cm² threshold.

We note, however, that the latter ESR assessment (2022),⁸ which reports on the 15μg/100cm² threshold, also acknowledges insufficient information to determine the health effects associated with this level of contamination (see page 1, final paragraph). This highlights the continuing uncertainty regarding the relationship between surface contamination levels and health outcomes.

It is also relevant to note that remediation or clearance levels adopted in other developed countries, including Australia, the United Kingdom and the USA, are substantially lower, generally ranging from approximately 0.05 to 1.5μg/100cm², compared with the 15μg/100cm² level used in New Zealand.

In addition, Van Dyke et al. (2014)⁹ reported dermal transfer efficiencies of methamphetamine ranging from approximately 11% for dry hands to 26% for wet hands, with transfer varying according to surface type. Based on these estimates, a contamination level of even 1.5μg/100cm² may not always be low enough to keep the amount of methamphetamine absorbed through the skin below levels that could cause adverse health effects. Therefore, more research is needed to understand the safe level of contamination and its potential health risks.

Data and methodology issues

Regarding the statistical analysis, we agree that it has several limitations. We did not have property-level data on methamphetamine consumption or use, and therefore we were unable to incorporate this variable into our model. If such data had been available, we would have considered including it in the analysis. We also acknowledge the other concerns raised, including the non-random nature of the data, the selective testing of properties before and after the 2018 policy change and the potential changes in testing patterns after 2018. These limitations are already acknowledged and discussed in the limitations section of our article (page 83).¹

Economic benefits and future research

Lastly, we agree that, in a fiscally constrained environment, decisions about public investment need to carefully consider costs and potential benefits. However, strengthening methamphetamine contamination standards could also be considered a preventive strategy that may help reduce future health-related costs.

We acknowledge that the potential economic benefits of tighter contamination limits have not been quantified in our study. A separate study could therefore examine whether stricter contamination standards could reduce methamphetamine exposure, healthcare use and associated costs, and estimate the potential economic benefits of such a prevention strategy. We believe this would be a valuable area for future research.

Response to letter B³

Undisclosed conflict of interest

We acknowledge that the employment relationship of the second author, Ushan Goonawardane, with Safe & Healthy Home Solutions should have been disclosed at the time of submission.

The research was completed in late 2024 to early 2025. At that time, Ushan was an unpaid intern at Safe & Healthy Home Solutions and a full-time student at Unitec Institute of Technology, assisting the first author with the research. The data used in the study were obtained from the company, and the analysis was conducted in good faith. Importantly, at the time the research was conducted, Ushan did not have an employment relationship with the company.

Subsequently, Ushan received a job offer from the same company and later became employed there. We acknowledge that this subsequent employment relationship creates a potential conflict of interest that should have been disclosed when the paper was submitted.

We wish to emphasise that the research was completed before Ushan received the job offer and before he commenced paid employment with the company, and that aside from providing the data, Safe & Healthy Home Solutions played no role in the research. Given the potential conflict of interest that subsequently arose, we have provided the New Zealand Medical Journal with Ushan’s updated conflict of interest form, along with his written apology acknowledging that this disclosure should have been made earlier.

Paper model and estimates

We respectfully clarify that our estimates represent associations, not causal effects. We have never stated that the fixed-effects model identifies the causal effect of the 2018 policy change, either explicitly or implicitly. The fixed-effects model was used to account for unobserved, time-invariant property characteristics. We acknowledge that this approach does not, by itself, separate the association with the policy change from other factors that may have changed contemporaneously, including changes in methamphetamine consumption. Therefore, we reiterate that the findings should be interpreted as associations observed following the 2018 policy change.

Health harms

We respectfully disagree with the suggestion that the potential harms associated with methamphetamine consumption and passive exposure are not sufficiently established in the literature. The adverse health effects of methamphetamine use and passive exposure are well documented. This is highlighted in the first paragraph on page 80 of our paper,¹ where we cite relevant supporting studies. We have provided more studies at the end of this response.^5–7,9

We hope readers will appreciate that not all of the studies cited in the literature have been subject to the criticisms referred to above.

Meth manufacturing

We acknowledge that the dataset reported by Bogun et al. (2023)¹⁰ ends in 2021. However, our discussion of the broader trend was not based solely on this study. We also presented another reference in Figure 1 showing that methamphetamine use in New Zealand continued to increase through October/November 2024.¹

Response to letter C⁴

Misrepresentation of the policy history

We would also like to clarify that we did not describe the 2018 Gluckman report as itself being a policy change.¹¹ Our point was that the report represented an important part of the policy development process and that, following the report, the threshold level used in the relevant policy framework was increased. Therefore, our intention was not to characterise the Gluckman report itself as the policy change or to misrepresent its purpose. We were referring to the change in the policy threshold that occurred following the report. Please read paragraph 1 on page 81 of our article.¹

Lack of engagement with major contrary evidence

The literature and reports included in the study were selected based on their relevance to the research questions and the policy context examined. We did not intentionally exclude evidence to support a predetermined conclusion. Regarding the more recent report by the ESR (now the PHFS), please see the citation issues heading in the response to letter A.

Transparency and reproducibility of the dataset

We respectfully disagree with the suggestion that the analysis lacks transparency or reproducibility. The analysis is fully reproducible, and the methodology used has been clearly described in the published article.

If the Editor would find it helpful, we would be very happy to provide the underlying dataset and the Stata codes used for the analysis for independent verification and replication.

Regarding the testing methodology, we provided a detailed explanation in the first paragraph on page 82 of the published article.¹ This section clearly describes the methodology and procedures used for testing the properties.

We therefore believe that the analysis is sufficiently transparent and reproducible.

Selection bias is potentially sufficient to explain the findings

We respectfully acknowledge the concern regarding voluntary testing and the potential for selection bias. However, we would like to clarify that we never stated in the article that our estimates represent causal effects. We reiterate that the results presented in the paper should be interpreted as associations, rather than causal estimates. We agree that the voluntary nature of testing may introduce potential selection bias, and we have explicitly acknowledged this issue in the limitations section on page 83 of the published article.¹

Health claims substantially exceed the evidence presented

We agree that our study does not contain data on human exposure, biological uptake, toxicology, health outcomes, healthcare utilisation or child health. We acknowledge that we did not estimate the direct health effects of methamphetamine contamination in residential properties, and we do not intend to suggest that our study provides such estimates. Please see the causal relationships and health impacts heading in the response to letter A for a further detailed response.

Failure to distinguish contamination from risk

We reiterate that we do not claim that an increase of 1.93μg of methamphetamine per 100cm² has a known or established health impact. Rather, we believe this is an important area for further research in New Zealand. Studies conducted in other countries have reported potential adverse effects associated with passive exposure.

In New Zealand, dedicated research is needed to determine whether increases in methamphetamine contamination are associated with any measurable health effects and, if so, what those effects may be. As noted in the last sentence of our article on page 84, we have identified this as an area requiring further investigation.¹ Subject to the availability of sufficient data and resources, we may undertake such research in the future.

We hope that our responses help to clarify the points and concerns raised. We trust that, with the information and explanations provided, New Zealand Medical Journal readers will now be better placed to consider the issues and make their own informed assessments.

Sincerely,
Muhammad Irfan, Ushan Indika Goonawardane, Craig Robertson and Michael P Cameron