Lung function changes in children exposed to mine fire smoke in infancy

Chronic, low‐intensity air pollution exposure has been consistently associated with reduced lung function throughout childhood. However, there is limited research regarding the implications of acute, high‐intensity air pollution exposure. We aimed to determine whether there were any associations between early life exposure to such an episode and lung growth trajectories.


INTRODUCTION
Globally, 93% ($1.8 billion) of children live in environments with air pollution levels above World Health Organization guidelines. 1 Air pollution is a risk factor for respiratory morbidity and mortality and, while the burden of disease is experienced throughout the general population, there are vulnerable individuals who are more susceptible to these detrimental health effects. 1,2Particulate matter (PM) with an aerodynamic diameter of less than 2.5 micrometres (PM 2.5 ) can bypass the upper airway defences and penetrate into the alveolar region of the lungs. 2,3The smallest particles in this size fraction can also traverse the lung epithelium and enter the circulation leading to systemic effects. 4arly life is a critical window of lung development and growth with considerable increases in lung volume and size. 5n early childhood, the lung parenchyma is rapidly growing, primarily through the addition of alveoli. 5This steep growth trajectory reaches a maximum in early adulthood before declining into senescence. 6Children are particularly vulnerable to the detrimental effects of air pollution exposure due to their higher breathing rates and less developed airways. 1 In addition, there is an accumulating body of evidence suggesting that chronic lung diseases originate in early life, due to deviations in normal lung growth trajectories. 6,7here are conflicting results related to the effects of early postnatal exposure to air pollution with one study reporting that early postnatal exposure to PM 2.5 is associated with a low growth trajectory of forced expiratory volume in first second (FEV 1 )/forced vital capacity (FVC) and reduced FEV 1 and FVC [8][9][10] while a similar study found no associations between postnatal exposure to PM 2.5 and FEV 1 or FVC. 11However, these studies focused on examining the effect of exposure to chronic, low-moderate levels of air pollution exposure and there is limited research regarding the consequences of an acute, high-intensity air pollution episodes on childhood lung function.In addition, most of these studies have been conducted in the context of chronic lifelong exposure to PM 2.5 which has made it difficult to separate the ongoing effects of post-natal exposure to PM, from the in utero effects.
In 2014, embers from a landscape fire caused coal in the Hazelwood coalmine (South-Eastern Australia) to ignite and burn continuously for 45 days. 12In Australia, the air quality standard for PM 2.5 is 25 μg/m 3 averaged over 24-h.Out of the 45 days the fire burned, our modelling suggests the PM 2.5 standard was exceeded on 23 days and reached a maximum PM 2.5 of 731 μg/m 3 in the town of Morwell. 12,13he Early-Life Follow-up (ELF) 14 stream of the Hazelwood Health Study (HHS) was established to assess the long-term health consequences of exposure to emissions from the fire in early life.We have previously shown that children exposed post-natally to emissions from this fire had mild impairments in peripheral lung mechanics, 3-years after the fire. 13However, it remains unclear whether these effects have persisted throughout childhood.
The aim of this study was to investigate the associations between acute early life exposure to PM 2.5 emitted from a coalmine fire and lung function changes 3-7 years after the fire.

Participants
The study participants originated from a clinical testing subsample of the longtitudinal ELF study.A detailed cohort profile is published elsewhere. 14In summary, children born between 1 March 2012 and 8 February 2014 in the Latrobe Valley (exposed postnatally) were recruited between February and September 2016.A participating parent/carer completed a baseline survey with sociodemographic, health and family information.Participants were invited to participate in clinical follow-up studies, 3-and 7-years after the coalmine fire.

Exposure estimates
Hourly PM 2.5 exposure concentrations from the coal mine fire were estimated using meteorological, dispersion and chemical transport modelling with a 1 Â 1 km spatial resolution as described previously. 12The model estimated PM 2.5 emission rates at hourly time intervals based on the area of coal burned and other emission factors. 12Individual exposure estimates were then calculated for each 24-h period, based on the location diaries completed by the parent/carer, to provide daily average mean and maximum PM 2.5 exposure estimates.

Lung function
Lung function was evaluated using respiratory oscillometry (TremoFlo C-100, Thorasys, Montreal, QC, Canada) according to European Respiratory Society (ERS) technical standards. 15Standardized Z-scores were calculated for resistance (R 5 ) and reactance (X 5 ) at a frequency of 5 Hz and the area under the reactance curve (AX). 16Standardized Z-scores account for differences in age, sex and height allowing comparison of children at various ages.Acceptability criteria of the measurements included no evidence of artefacts, and three to five acceptable measurements with a coefficient of variation of <10% for each child.

SUMMARY AT A GLANCE
The Hazelwood coalmine fire (Victoria, Australia) was an extreme air pollution episode that lasted for 6-weeks leading to community concerns regarding the potential health effects.We found no evidence of ongoing lung function changes in children exposed in infancy.Previously reported deficits in lung function appeared to have resolved over time.

Statistical analysis
Two sets of analyses were conducted to address the aim of the study.We have previously shown a cross-sectional relationship between post-natal exposure to PM 2.5 from the coal mine fire and lung function at the 3-year follow-up. 13On this basis, in the first analysis, we assessed whether there was a cross-sectional relationship between post-natal exposure to PM 2.5 and lung function at the 7-year follow-up.In the second analysis, we determined whether there was any association between post-natal exposure to PM 2.5 and lung function trajectories between the 3-and 7-year follow-ups.
For the cross-sectional analysis, we fitted linear regression models to evaluate the association between exposure to mine fire PM and lung Z-scores (R 5 , X 5 and AX).In subsequent covariance-adjusted models, a range of a priori covariates were included based on previous work (see Appendix S1 in the Supporting Information).
For the longitudinal analysis, we fitted linear mixedeffects regression models with random-intercepts to evaluate changes in lung function over time attributable to mine fire exposure.To facilitate interpretation, we also reported the estimated exposure effects from mine fire PM 2.5 in both follow-ups.The exposure effect in the 3-year follow-up was estimated directly from the linear mixed-effect regression model as the coefficient for the exposure variable.The exposure effect, in the 7-year follow-up was estimated using the linear combination of the coefficient of the exposure variable Flowchart of participants through the study.
F I G U R E 2 Lung function measurements, R 5 , X 5 and AX, from the 3-to 7-year follow-up.
LUNG FUNCTION CHANGES AND AIR POLLUTION and the interaction coefficient of the exposure variable and the follow-up time point variable (using R package mitml version 0.4.3).To ensure the robustness of the finding, we estimated the exposure effects in two sets of models: (1) base models, which only included the exposure variable, the time point variable and the interaction between them and (2) covariance-adjusted models, which also included confounding factors.The exposure estimate was investigated in two forms, as mean PM 2.5 in increments of 10 μg/m 3 and maximum PM 2.5 in increments of 100 μg/m 3 .
Multiple imputation by chained equations (MICE using the mice R package version 3.14.0)with predicted mean matching was used to address missing data in outcome variables and covariates.Twenty imputed datasets were used in stepwise variable selection as well as final analysis.Covariates were selected if they were retained in 80% of stepwise models across imputed datasets and pooled using Rubin's rule. 17 Studio version 4.1.3was used for the statistical analysis (see Table S4 in the Supporting Information for full list of R packages).Summary data are reported as means (SD) or median with ranges.Beta (β)-coefficients, 95% confidence intervals [95% CI] and p values are reported for all regression analyses.

RESULTS
PM 2.5 and lung function in the 7-year follow-up

Participant characteristics
Of the 79 children who were postnatally exposed and attended clinical testing in the 7-year follow-up, 74 had acceptable lung measurements according to ERS criteria (Figures 1 and 2).
There was a reasonable gender balance of children with 45% females, while 32% of the children were overweight or obese (Table 1).The sub-sample of children who attended clinical testing had some characteristics that differed from the remainder of the cohort.They had a younger gestational age and a higher rate of breastfeeding duration over 3 months compared to children who did not attend (Table S1 in the Supporting Information).This sub-sample of children also had a relatively high socioeconomic index, with 58% of mothers reporting an education beyond year 12 (Table S1 in the Supporting Information).Mean and maximum mine fire-related PM 2.5 exposure for the 74 children in this analysis had a median [Interquartile range, IQR] of 3.2 [2.1-11.5]μg/m 3 and 69.3 [34.9-149.8]μg/m 3 , respectively (Table 1).

PM 2.5 and lung function in the 7-year follow-up
There were no statistically significant univariate relationships between mine fire-related PM 2.5 and any of the lung Z-score measures ( p > 0.07 for all comparisons, Table 2).
The associations between mine fire-related PM 2.5 and R 5 , X 5 and AX were not altered by adjustment for covariates (p > 0.06 for all comparisons, Table 2) and there was minimal effect on the precision of the estimates.However, in the covariance-adjusted model for maximum PM 2.5 there was a modest improvement in X 5 (β: À0.18 [0.36, À0.01] p = 0.045, Table 2).PM 2.5 and lung function trajectories from the 3-to 7-year follow-up

Participant characteristics
Of the 101 children who were postnatally exposed and attended clinical testing during the 3-year follow-up, 84 had acceptable lung function measurements according to the ERS standard (Figures 1 and 2).There were 115 children who attended the 3-and/or 7-year follow-up, with 60 attending in both years.
The gender balance, age and covariate characteristics of postnatally exposed children who attended in either the 3or 7-year follow-up was not considerably different from children who never participated (Table S2 and S3 in the Supporting Information).

PM 2.5 and lung function trajectories
There were improvements in lung function Z scores from the 3-to 7-year follow-up that were associated with PM 2.5 exposure (see Table 2).This was evident in both X 5 (β: À0.37 [À0.64,À0.10] p = 0.009, Table 2 and Figure 3) T A B L E 2 Mixed-effect model for linear regression analysis, reported as base models and covariance-adjusted models.
Changes in lung Z-scores attributable to mine fire PM  2 and Figure 3).In the covariance-adjusted model, the strength and precision of the estimates did not change considerably (p = 0.026, Table 2).There were no associations between PM 2.5 exposure and R 5 from the 3-to 7-year follow-up in either the unadjusted or covariance-adjusted models with inclusion of mean PM 2.5 (p > 0.16, Table 2).
A similar pattern of improvement from the 3-to 7-year follow-up that was associated with maximum PM 2.5 was observed for both X 5 (β: À0.22 [À0.42,À0.02] p = 0.032, Table 2 and Figure 3) and AX (β: À0.23 [0.47, 0.01] p = 0.059, Table 2 and Figure 3).In the covariance-adjusted model the strength and precision of these estimates did not change considerably, however, they did fall below our significance threshold of p < 0.05.

DISCUSSION
This study evaluated the long-term effects of postnatal exposure to fire-related PM 2.5 on childhood lung function.There were no associations observed between lung function and postnatal exposure to fire-related PM 2.5 in the 7-year followup.However, there were improvements in peripheral lung function (X 5 and AX) from the 3-to 7-year follow-up.This observation suggests that the mild effects demonstrated in our previous publication on the cross-sectional analysis of 3-year follow-up data 13 did not persist to the 7-year follow-up.
There is a growing body of literature regarding the impacts of early life exposure to PM 2.5 on lung function, 18,19 and the link between childhood exposure to PM 2.5 and lung infections and diseases in adulthood. 20,21However, the health consequences of acute, high-intensity air pollution exposure in early life are poorly understood while data on chronic exposure are conflicting.For example, a prospective cohort study conducted in Australia found early postnatal exposure to PM 2.5 was associated with a very low trajectory of FEV 1 /FVC in childhood. 8Similarly, a study conducted in the Netherlands found an association between postnatal PM 2.5 and reduced FEV 1 growth. 9While a Chinese study found associations between chronic exposure to air pollution and reduced FVC growth. 10In contrast, a similar study conducted in Japan found no associations between postnatal exposure to PM 2.5 and FEV 1 or FVC. 11evelopment of the lungs relies on highly coordinated growth to ensure this complex innervated, ventilated and vascularised structure functions in the appropriate way. 22I G U R E 3 Coefficient plot of mixed-effect linear regression analysis attributable to mean PM 2.5 .Exploring the relationship between lung function of children exposed postnatally to mine fire smoke over time.Estimates from panel A refer to the base model without any covariates and estimates from panel B refer to the covariance-adjusted model with the inclusion of selected covariates.
Lung growth changes follow a distinct pattern of rapid growth before declining into senescence in early adulthood. 23The determinants driving these distinct lung changes included natural decline over time due to the aging process [23][24][25] and a number of childhood risk factors including maternal smoking, 7,26 and air pollution. 18,194][25] Research regarding the health implications of air pollution exposure in early childhood has focused on chronic, low-intensity air pollution from urban settings. 10,18The ELF Study is one of the first to investigate the health implications of early life exposure to air pollution on lung function throughout childhood.The first clinical follow-up, 3-years after the fire, found a modest association between postnatal exposure to firerelated PM 2.5 and reduced respiratory reactance. 13However, the present study, 7-years after the fire, found no detectable relationships between postnatal exposure to fire-related PM 2.5 and lung function, and overall improvements in lung function from the 3-to 7-year follow-up.
There are a number of plausible explanations for the results observed.Firstly, there may have been short-term changes to lung reactance 3-years after the fire 13 but the structural impact was minor, such that the lungs were able catch-up to the normal changes 4 years later. 7Prior to adulthood, as the lungs grow in volume, the magnitude of respiratory reactance decreases. 27As described by Bui et al., lung growth changes follow a number of distinct patterns including the pattern observed in the current study of early low lung function followed by accelerated growth. 7During this period of rapid development and growth the lungs increase in size, undergo alveolarization and elastin fibre maturation all of which contribute to the elasticity of the lung, a major component of respiratory reactance. 28,29AX can be considered a combined measure of respiratory elastance and peripheral inhomogeneity. 28,29The small highly compliant airways have increased tendency to collapse resulting in a greater susceptibility to airway obstruction when at a young age. 28,29However, it is worth noting that the current study was conducted well before the peak in lung function and it is plausible that the early effects observed in this cohort could still lead to accelerated lung function decline later in life with exposure to environmental insults throughout life.Further measurements at later timepoints would clarify our understanding of the influence of environmental exposure on lung growth changes.
Another plausible explanation lies in the high intraindividual variability in lung development in mid-late childhood. 30Cohort studies that measured lung function in early adolescence found associations between early life exposure to air pollution and lung function. 8,9The next round of testing in this cohort, will align with the children entering adolescence and there will be an opportunity to more clearly observe the potential effects of early life exposure to PM 2.5 and other childhood risk factors on lung function.
It is important to acknowledge that there were some differences between children that attended the 3-year follow-up compared to those that attended the 7-year follow-up.There were differences in the prevalence of cold and flus and medication usage in the 24-h prior to clinical assessment from the 3-to 7-year follow-up, although the latter were typically vitamins or melatonin tablets rather than medications for respiratory conditions.While these could conceivably influence lung function, we found no association between these factors and lung function outcomes in our previous study, nor were these factors retained in the stepwise multiple imputation models indicating that there were not linked to our outcome measures. 13There were also differences in the gestational age from the 3-to 7-year follow-up, however, while statistically significant, the magnitude of the difference was such that this is unlikely to be physiologically significant.Maternal smoking during pregnancy is a risk factor for respiratory decline and children that attended the 7-year follow-up had fewer mothers that smoked during pregnancy compared to the previous followup.However, this was not statistically significant and was included in the covariance-adjusted models.
This study includes a number of strengths such as accurate exposure classification given the clear time-interval of the exposure event.We also had exposure estimates calculated for each individual child based on exposure models and the child's location every 12-h during the fire.We were able to directly measure lung function at multiple time points using a non-invasive technique.Z-scores allowed comparison of different children of different genders and heights, both of which are highly associated with lung function.However, it is important to note that the sample size was limited, which may have implications for the power of the study, however, it was sufficient to detect a significant improvement in lung function between follow-ups.
Overall, there were no associations between postnatal exposure to fire-related PM 2.5 and lung function, 7-years later.Consistent with catch up lung growth, we found evidence that the lung function deficits in children exposed to higher levels of PM 2.5 did not persist over time.This suggests that postnatal exposure to the levels of mine firerelated air pollution, on the background of low chronic exposure, were not sufficient to cause permanent lung function changes in children.Further studies are required to determine whether this remains true across the lifecourse.
Participant characteristics in 2017 and 2021.
T A B L E 1Note: Differences between groups were assessed using T-tests for continuous variables and Fishers exact test for binary variable.Included children (n = 60) who attended both clinics.Bold values indicate p < 0.05.