Exposure to secondhand smoking (SHS) in children is a serious health concern that requires further attention, as it is the most frequent indoor air pollutant [1
]. SHS is the third leading cause of preventable death worldwide after smoking and alcohol, and therefore a quarter of children worldwide have been exposed to SHS [2
]. Longitudinal studies have shown that maternal smoking during pregnancy had higher negative cognitive and behavioral outcomes in children [3
]. Additionally, mothers who smoked one cigarette packet a day during pregnancy have children whose IQ is 2.87 points lower than children of non-smoking mothers [4
]. Furthermore, mothers who are exposed to secondhand smoking have children who are more likely to have attention deficit disorder, conduct disorder, aggression, depression, and hyperactivity [5
There is a growing body of evidence that demonstrates that smoking releases harmful toxicants that lead not only to adverse behavioural and cognitive effects, but also an increase in cardiovascular diseases, chronic obstructive pulmonary disease, and many forms of cancers [7
]. Heavy metals can also have adverse effects on kidney function [10
] as well as on the hepatic system [11
]. Cigarette smoke contains toxicants that gets inhaled or absorbed by people exposed [12
]. Certain heavy metals, such as chromium (Cr), lead (Pb), cadmium (Cd), copper (Cu), aluminum (Al), and nickel (Ni) have shown to be higher in smokers [13
]. These metals are known to have lifelong effects on health, while others have an immediate effect, depending on the doses and type of metal [14
]. For example, Pb is a risk factor for cardiovascular diseases at much lower levels and kidney damage at very high levels [15
]. Furthermore, mental retardation can occur at exposures to higher levels of heavy metals and neurocognitive disorders such as impaired memory and lower IQ at much lower exposures [11
]. In children, the effect of SHS is further exacerbated by the fact that they are still growing and hence absorb heavy metals at a greater rate than adults. Children whose parents smoke have double the risk of lower respiratory illnesses like bronchitis and pneumonia compared to those from parents who do not smoke [16
]. Moreover, the more the parents or family members smoke around their children, the more likely these children are to have adverse health effects such as respiratory diseases, ear infections, and severe asthma attacks. Being exposed to SHS can also affect children’s development and behavior. As such, exposed children may have trouble paying attention, compromising their full potential in their learning capabilities at school [17
The heavy metals can be measured in nails with a window of exposure to between 1–6 months depending on how long the nail is [19
]. Metals can be deposited in the nails after being either ingested or inhaled and metabolized (within 1 to 2 months). As such, nails provide a longer integration of exposure to heavy metals [20
]. Nail samples, particularly toenails, are also less exposed to external contamination [20
]. Just like hair, they are simple to collect, easy to analyze, and store well [21
]. Also, elements deposited into the nails are not subject to additional metabolic processes and many elements are present in the nail at substantially larger concentrations than in urine or blood [22
]. There have been studies that have examined the reliability of toenail measurements [23
]. Although there may be underestimation of the correlation of association with other measures due to measurement errors, the toenail concentrations of most elements are still suggested to be useful biomarkers of exposure in which a single sample is assumed to represent long-term exposure [23
Overall, smoking rates in New Zealand (NZ) are decreasing (13% of adults smoke, as compared to 25% in 1996–1997), however, Pacific Island people still have the second highest smoking rates after Māori (23%) [24
]. Exposure to SHS in the home has almost halved between 2006–2007 and 2012–2013 for New Zealand adults (7.5% to 3.7%, respectively) and for children aged 0–14 years (9.6% to 5%, respectively) [25
]. There was a decline in the proportion of Pacific students exposed to smoke inside homes, from 31 percent in 2001 to 25 percent in 2006 [26
]. Even though significant progress has been made in reducing the SHS exposure, non-smokers in NZ remain exposed, with the highest exposure being in pre- and school-aged children, Māori, and those of low socio–economic status [27
]. A recent finding within the Pacific Island Family Cohort study at the 11-year follow-up showed a high prevalence of smoking in the fathers [28
]. As such, smoking is still the leading contributor to death within the Pacific population [29
This is the first study that aimed to explore SHS in Pacific Island children and its association with toenail heavy metals concentration. Therefore, the aim of this study was to assess the possible association between SHS and toxic metal concentrations of chromium (Cr), lead (Pb), cadmium (Cd), copper (Cu), nickel (Ni), and aluminum (Al) in nine-year-old Pacific Island children who were living in South Auckland, New Zealand.
2. Materials and Methods
2.1. Study Area and Population
An observational study was undertaken between July 2010 and July 2011 involving a sub-sample of nine-year-old Pacific children within a Pacific Island Families (PIF) longitudinal cohort in which these individuals have been followed since birth [30
]. These children were born in Middlemore hospital, South Auckland, New Zealand, to mothers in the year 2000 [30
]. The eligibility criteria for the core PIF study was that at least one parent of the new-born child identified themselves as being of Pacific Island ethnicity and was a permanent resident of New Zealand. South Auckland has the highest number of Pacific Island population in New Zealand [31
] from which this cohort was selected. The participants who were healthy were enrolled into this sub-study at the same time as they were recruited into the PIF core study. Consent and assent was sought from children and their mothers before commencing this study. Children who provided assent but had short nails (typically less than 0.05 g (g) of cut material) were excluded. This study was approved by the NZ health and disability ethics committee (NTX/07/05/050).
2.2. Heavy Metal Assessment
Toenails were used as a matrix for detecting heavy metals which included chromium (Cr), lead (Pb), cadmium (Cd), copper (Cu), nickel (Ni), and aluminum (Al). Toenail sample collections were conducted in the school setting. Approximately > 50 milligram (mg) of toenail clippings from all toes were collected. These were then placed in a sealed zip-lock polythene bag and each sample was identified with a unique code identification number to ensure anonymity when analyzed. The samples were then stored at room temperature until laboratory analysis.
Toenail Sample Laboratory Analysis
There were different steps employed for the chemical analysis.
Step 1: Digestion of toenails: All potential external contaminants in toenails, such as cosmetic treatments and “dirt” were washed prior. The washing procedure involved: (1) five steps using acetone, deionized distilled water (DDW, 18.2 MΩ) (x3) then acetone again; (2) at each washing step enough liquid was added to cover the sample and sonication (ranssonic water bath (T460/H)) for 5–10 min; and (3) decantation. Following the washing procedure, the nail samples were dried overnight at 60 °C in a drying oven (LTE Scientific). Samples that did not have enough toenails were further processed using a higher dilution factor.
Step 2: Chemical Analysis: The washed and processed nail samples was then analyzed in the Agilent 7700 x ICP-MS instrument (Agilent Technology, Santa Clara, CA, USA). The certified reference material (CRM) used were NIST SRM 1643e (National Institute of Standards and Technology, Gaithersburg, MD, USA) and TMDA-54.4 (National Water Research Institute, Canada). The recovery rate range for all the elements was between 76% to 101.3%. All instrumental data for each element (according to the isotope selected) was reported as counts per second. The value was corrected for a reagent blank signal (to correct for any contribution from the digestion procedure) and ratioed with the internal standard isotope value (to correct any instrumental drift or signal enhancement/depression caused by the matrix). Data for the calibration standards was handled in the same manner and an Excel™ calibration curve produced for each element, with ratio signal (y-axis) and concentration of five standards (x-axis), from which the calibration equation was determined for calculation of the unknown toenail sample elemental concentration. The elemental values for each toenail sample were corrected for the dilution factor and the final values used in this data analysis.
Validated and reliable questionnaires were administered at the nine-year phase to both mother and child. These questionnaires were interviewer-administered to collect information on socio–demographic, cultural, environmental, child development, family and household dynamics, lifestyle, and health issues. Participant characteristics and demographic variables which were of interest for this study were included in the analysis.
Demographics: Child’s gender (girls, boys), child’s Pacific ethnicity, including Samoans, Tongan, Cook Island, and Others group (which included European, Māori, and Niuean), as determined by the mother at age two years phase, household income levels (categorized into $0–$20,000, $20,001–$40,000, > $ 40,000), marital status (non-partnered, partnered, defacto, partnered, legally married).
Lifestyle: Household smoking status (yes, no), number of smokers in households. Secondhand smoking (SHS) was assessed by determining households that smoked versus households that did not smoke.
Health issues: General health (poor, very good), allergies (yes, no), asthma (yes, no).
Weight of children: The average weight and height of children were calculated according to the procedures documented in an operation manual from which the International Obesity Task Force (IOTF) [32
] criteria was derived. Prior to data collection, equipment was standardized before weight and height data were collected.
2.4. Statistical Analysis
All data which were obtained for this sub-study were stored in Microsoft AccessTM and ExcelTM (Microsoft Corporation, Redmond, WA, USA) template. The data was edited, range and consistency checks were performed, and the data was coded for analysis where necessary. Questions with no response received a distinct code and were not included in the analysis. The edited data was then exported to a statistical software using SAS version 9.4. (SAS Institute Inc., Cary, NC, USA).
Data were expressed as median with interquartile range for continuous variables and categorical variables were presented as counts and proportions. Mean and standard deviation was also calculated for comparison with published values. The Chi-square test was used to test for association across the demographics variables by those who smoked in the household versus those who did not. Because the variables were not distributed normally, nonparametric statistics were used throughout the study. The between-group differences in the elements were analyzed using the Mann–Whitney U test. The Spearman rank test were used to determine correlations between the elements and number of smokers in household. All p values were two-tailed, and p < 0.05 was considered statistically significant.