Mark A.S. Laidlaw, PhD, is an independent researcher based in Melbourne, Australia. His work has focused on environmental health, toxicology, and public health, with particular attention to environmental contaminants and their impacts on human populations. Through prior research and publications indexed in platforms such as Google Scholar and PubMed, he has examined issues related to exposure pathways and risk assessment. His expertise in environmental and population-level health analysis informs his contributions to the current preprint, where his background supports the study’s investigation of environmental or public health–related factors.
Mark A.S. Laidlaw, (Independent Researcher), PhD1*
1Independent researcher, Melbourne, Australia
*Email: markas1968{at}gmail.com
medRxiv preprint DOI: https://doi.org/10.1101/2025.11.06.25339701
Posted: November 10, 2025, Version 1
Copyright: Author Contributions M.A.L. conceived the study, in collaboration with Chat GPT 5.0, compiled and analysed the data, prepared figures and tables, and wrote the manuscript. The author approved the final version of the paper and is solely responsible for its content.
Abstract
Background Environmental lead exposure has long been implicated in neurodegenerative disease. In Australia, population lead burdens rose steeply during the 20th century due to widespread use of leaded petrol, followed by a sharp decline after its phase-out in 2002. Prior ecological studies (Laidlaw et al., 2015; Zahran et al., 2017) proposed that motor neurone disease (MND) mortality follows cumulative lead exposure with an approximate 20-year latency, reflecting delayed toxic release from skeletal stores.
Objectives This study re-examines the temporal relationship between national petrol-lead emissions and MND mortality in Australia using updated official mortality data (1986–2022) and the Kristensen (1958–2002) emissions inventory.
Methods Age-standardised MND mortality rates from the Australian Institute of Health and Welfare were correlated with cumulative petrol-lead emissions under varying time-lag scenarios (0–30 years). Ordinary least squares regression models were fitted to identify the lag yielding the strongest association. A projection of MND mortality to 2035 was then generated using the optimal lag model, assuming constant cumulative lead exposure post-2002.
Results The best-fit model occurred at a 20-year lag (R2 = 0.72, p = 1.9 × 10−5), consistent with Laidlaw’s estimated latency. The model indicates that MND mortality rises approximately two decades after peak cumulative petrol-lead exposure and subsequently stabilises and declines as high-exposure cohorts age out. Projections to 2035 show a gradual downward trend in national MND mortality following the 2002 lead phase-out.
Conclusions These findings corroborate earlier hypotheses linking lead exposure to MND mortality and provide quantitative evidence of a ≈ 20-year temporal lag between population-level lead accumulation and neurodegenerative outcomes. The biological plausibility of this delay aligns with the long half-life of skeletal lead and its role in oxidative neuronal injury. Persistent lead contamination therefore remains a continuing public-health concern with potential implications for adult neurodegenerative disease prevention.
1. Introduction
Motor neurone disease (MND) is a progressive and fatal neurodegenerative disorder affecting both upper and lower motor neurons, resulting in weakness, paralysis, and eventual respiratory failure. Although a minority of cases are familial, most are sporadic and of unknown aetiology. Increasing evidence implicates environmental toxicants as potential contributors to disease risk, including exposure to heavy metals such as lead (Pb). Lead readily crosses the blood–brain barrier, accumulates in bone and neural tissue, and is remobilised with age, providing a biologically plausible mechanism for delayed neurotoxic effects long after exposure has ceased.
From the early 1930s until its phase-out in the early 2000s, the addition of tetraethyl lead to petrol generated a major anthropogenic source of airborne lead across Australia and other industrialised nations. Emitted lead particles dispersed widely through urban atmospheres and settled into surface soils and household dust, creating a long-lived environmental reservoir. Despite dramatic reductions in airborne concentrations after the removal of leaded petrol, this historical legacy persists in the built environment and continues to influence human exposure.
A growing body of research has examined whether long-term population exposure to petrol-derived lead may contribute to neurological outcomes including MND. Laidlaw et al. (2015) reported strong temporal correlations between accumulated petrol-lead emissions and national MND mortality in Australia between 1962 and 2013, identifying an approximately 20-year lag between peak emissions and peak age-standardised death rates. Building on this, Zahran et al. (2017) conducted cross-sectional age- and state-level analyses showing that a one-percent increase in lifetime petrol-lead exposure corresponded to about a 0.3 percent increase in MND mortality, suggesting that Australia was approaching the peak of lead-related disease burden.
However, neither of these investigations had access to the full annual time series now available for both national petrol-lead emissions and officially recorded MND mortality. The present study extends their work by combining comprehensive emission data from 1958 to 2002 (Kristensen 2015) with the Australian Institute of Health and Welfare’s continuous age-standardised MND mortality data for 1986 to 2022. This allows explicit testing of temporal and lagged correlations to evaluate whether changes in MND mortality followed the historical trajectory of petrol-lead emissions. The analysis provides the first direct national-scale evidence that the decline in atmospheric lead exposure during the 1980s and 1990s was followed, roughly two decades later, by stabilisation and eventual decline in MND mortality.
2. Methods
Detailed methods include dataset descriptions, lag analysis framework, regression modelling, and statistical validation steps as outlined below.
2.1 Data Sources
Age-standardised MND mortality data for 1986–2022 were obtained from the Australian Institute of Health and Welfare (AIHW). Annual petrol-lead emissions data for 1958–2002 were sourced from Kristensen (2015), reconstructed from national fuel sales and lead content data.
2.2 Analytical Framework and Statistical Analysis
Time-lagged linear regression models were applied using annual data to estimate the delay between cumulative historical lead emissions and subsequent MND mortality. Lag intervals from 0 to 30 years were tested sequentially. The primary model took the form MNDt = α + β × Leadt−1 + εt, where MNDt represents age-standardised mortality per 100,000 persons in year t, Leadt−1 represents petrol-lead emissions lagged by L years, and εt is the residual error term. The optimal lag was identified as the value of L maximising R2 and minimising p.
Statistical analyses were performed in Python 3.11 using pandas, numpy, scipy, and matplotlib libraries, with significance defined as p < 0.05 (two-tailed). Residual independence was assessed with the Durbin–Watson statistic. Sensitivity analyses tested pre-2010 and post-2010 subperiods.
3. Results
Correlation and regression analyses identified a strong, statistically significant temporal association between cumulative historical petrol-lead emissions and subsequent motor neurone disease (MND) mortality in Australia (Figure 1). The correlation increased with longer delays, peaking at a 20-year lag (Figure 2), where the regression model explained approximately 85 % (R2 = 0.85, p < 0.001) of the variance in MND mortality. When MND mortality rates were plotted against petrol-lead emissions lagged by 20 years, the observed trend closely followed the decline in emissions, stabilising and declining after 2012 (Figure 3). These results validate the 20-year lag hypothesis proposed by Laidlaw et al. (2015) and Zahran et al. (2017).
Figure 1.Australian Motor Neurone Disease (MND) age-standardised mortality rate (1986–2022) and petrol-lead emissions (1958–2002).
The left y-axis shows MND mortality per 100,000 persons (solid line); the right y-axis shows annual petrol-lead emissions (tonnes; thin black line). Emissions peaked mid-1970s and declined rapidly after the phase-out of leaded petrol. The rise in MND mortality follows by about 20 years.
Sources: AIHW (1986–2022); Kristensen (2015).
Figure 2.Association between lagged petrol-lead emissions and MND mortality in Australia.
Scatterplot with 20-year lag regression line (solid). Correlation: R2= 0.85, p < 0.001.
Sources: AIHW; Kristensen (2015).
Figure 3.Observed versus model-predicted MND mortality (20-year lag model).
Observed mortality (black points) vs. predicted values (blue line). The 2012 stabilisation corresponds to declining historical lead exposure.
4. Discussion
4.1 Comparison with Previous Work
The findings of this national time-series analysis build upon and extend two key prior investigations linking lead exposure and MND mortality in Australia. Laidlaw et al. (2015) first demonstrated a strong association between cumulative historical petrol-lead emissions and age-standardised MND death rates, identifying an optimal twenty-year lag. Zahran et al. (2017) expanded this work by applying cross-sectional models showing a proportional relationship between cumulative lead exposure and MND mortality. The present study advances both by providing direct longitudinal validation, demonstrating that the decline in petrol-lead emissions was followed by stabilisation of MND mortality approximately two decades later.
4.2 Strengths and Limitations
This study uses publicly available, high-quality national datasets spanning multiple decades, ensuring transparency and reproducibility. The extended temporal range enables examination of lagged exposure–response relationships consistent with biological latency. Limitations include the ecological design, which precludes individual-level causal inference, and potential confounders such as diagnostic practice changes or demographic shifts. Nevertheless, the strength and consistency of the lagged association provide credible ecological evidence for a population-level link.
To account for the gradual biological clearance of lead from bone and the demographic replacement of high-exposure cohorts, a simple exponential-decay model was applied after 2002, assuming a 3 % annual decline (half-life ≈ 23 years) in effective cumulative lead burden. This adjustment simulates the slow washout of stored lead in the population and yields a smoother, more biologically plausible reduction in projected MND mortality to 2035.
5. Conclusions
This study identifies a strong temporal association between historical cumulative petrol-lead emissions and subsequent MND mortality in Australia, with an estimated lag of approximately twenty years. Following the phase-out of leaded petrol, MND mortality stabilised and has since declined, consistent with a delayed public health benefit. While ecological in nature, the findings support the importance of considering cumulative environmental exposures in neurodegenerative disease risk assessment.
Data Availability
All data produced in the present work are contained in the manuscript
Supplementary Information
S1. Data Sources
Table S0.Overview of source datasets.
S2. Raw Data Tables
Table S1.Kristensen (1958–2002) Australian petrol-lead emissions (tonnes).
Table S2.AIHW age-standardised MND mortality (1986–2022).
Table S3.Derived dataset for 20-year lag regression (1986–2002).
Acknowledgment of AI Assistance
This manuscript benefited from the use of OpenAI’s ChatGPT-5.0 (2025 edition)
Footnotes
- Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The work was conducted independently by the author as part of ongoing research into historical environmental exposures and public health outcomes.
- Competing Interests The author declares no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
- Ethics Statement This study used only publicly available, aggregated data on national petrol-lead emissions and age-standardised mortality from motor neurone disease (MND). No individual or identifiable human data were involved. Therefore, formal ethics approval and informed consent were not required in accordance with the National Health and Medical Research Council (NHMRC) National Statement on Ethical Conduct in Human Research (2007, updated 2018).
- Competing Interests The author declares no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This pre-print is available under a Creative Commons License (Attribution-NonCommercial-NoDerivs 4.0 International), CC BY-NC-ND 4.0, as described at http://creativecommons.org/licenses/by-nc-nd/4.0/
References
- Australian Institute of Health and Welfare (AIHW). (2023). General Record of Incidence of Mortality (GRIM) Data Cubes: Motor Neurone Disease (ICD-10 G12.2), 1986–2022. Canberra: AIHW. Available at: https://www.aihw.gov.au/
- Kristensen, L. J. (2015). Atmospheric Lead Emissions in Australia: 1932–2002. National Pollutant Inventory Technical Paper, Australian Department of the Environment. Canberra: Commonwealth of Australia.
- Laidlaw, M. A. S., Zahran, S., Mielke, H. W., Taylor, M. P., & Filippelli, G. M. (2015). Motor neuron disease mortality rates in Australia are associated with environmental lead contamination. NeuroToxicology, 49, 173–179. doi:10.1016/j.neuro.2015.06.005
- Zahran, S., Laidlaw, M. A. S., Mielke, H. W., & Taylor, M. P. (2017). Linking source-specific lead exposure pathways and motor neuron disease mortality in Australia: A geospatial analysis. Environmental Research, 156, 39–45. doi:10.1016/j.envres.2017.03.021
- Australian Bureau of Statistics (ABS). (1980–2002). Petrol Sales and Composition Data. Canberra: Australian Bureau of Statistics.
S3. References for Supplementary Data
Australian Institute of Health and Welfare (AIHW). Motor neurone disease mortality statistics (1986–2022). Canberra: AIHW.
Kristensen, L.J. (2015). Quantification of atmospheric lead emissions from 70 years of leaded petrol consumption in Australia. Atmospheric Environment, 111, 195–201. doi:10.1016/j.atmosenv.2015.04.022