Int. Med J Vol. 4 No 1 June 2005 

LEAD AND OTHER RISK FACTORS INFLUENCING HEARING IMPAIRMENT AMONG URBAN SCHOOL CHILDREN

Zailina Hashim[a] [b],  Chua Swee Kim[c], Noor Hassim Ismail[d],  Jamal Hisham Hashim[e] 

ABSTRACT

Background: This study was carried out to determine if lead and other risk factors influence the hearing impairment of primary school children in Kuala Lumpur, Malaysia

Methodology: A total of 215 children, 105 from School A and 110 from School B were recruited through stratified random sampling according to sex and age. A personal questionnaire interview was conducted to gather information on the subject's background, socioeconomic status, antenatal and childhood medical history as well as their house environment and daily activities. Audiometric tests were administered on these children. Blood lead was analyzed using Graphite Furnace Atomic Absorption Spectrophotometer.

Results: In School A, the prevalence of high frequency hearing impairment (HFHI) is 12..4% and low frequency hearing impairment (LFHI) was 9.5%. In School B children, the prevalence of HFHI is 21.7% and LFHI is 16.4%. School B children have significantly higher prevalence of HFHI and LFHI. The mean blood lead (PbB) of School A children (3.44 mg/dl) was significantly higher than School B children (3.16 mg/dl). The medical history influences the low frequencies hearing thresholds of both ears in all the studied children. Beside this, their daily activities also have unilateral effect on the right ears at high frequencies. However, specifically for School A children, significant relationship between blood lead with the high and low frequencies hearing thresholds  in the left ears, were also found.

Conclusion: All children's low and high frequencies hearing thresholds bilaterally, were mainly influenced by their poor medical history and unilaterally (left) were affected by noise from their daily activities. However, for School A children, very low blood lead affects high and low frequencies hearing thresholds unilaterally (left).

Keyword:  children's blood lead, hearing thresholds, high frequency hearing impairment, low frequency hearing impairment, medical history.

INTRODUCTION

Causes of hearing loss in children include ear infection (otitis media), congenital causes and acquired causes such as infection of measles, chicken pox, influenza, mumps, meningitis, head injury and noise or pollution exposure. Noise exposure would be the traffic noise, loud music from television or radio and noise from their toys. Urban air pollution from combustion of gasoline in motor vehicle engine contain particulate matter containing lead compounds.

 Lead is a well-known neurotoxic agent that may cause severe impairment of nerve tissue, particularly in the developing central nervous system (CNS). Lead-induced impairment of the auditory brain and cochlea is believed to contribute substantially to the cognitive disorders and learning disabilities associated with childhood plumbism¹. Pb exposed children and occupationally Pb exposed adults develop auditory brainstem abnormalities and significant hearing loss ^{2,3,4 ,5} .

A study ⁶ found that the urban lead concentration was about 16 times higher than the suburban areas. The dilute acid soluble fraction most likely contain halogenated species emitted from the combustion of leaded gasoline in motor vehicles. Therefore, children who live and attend schools in the urban areas are most likely to be highly exposed not only to noise but also to atmospheric lead ^{7, 8}. The objective of this study is to determine if lead and other risk factors which influence the hearing impairment of primary school children in the urban areas of  Kuala Lumpur.

METHODOLOGY

This investigation was carried out in 2 primary schools (School A and School B) in Kuala Lumpur. A total of 215 school children which made up of 105 children from School A and 110 children from School B were selected stratified in proportion according to class and sex from the name lists obtained from the class teacher. These children were in the Year 1 and 2, and were in the age range of 6^1/2 through 8^1/2 years. Inclusion criteria for this study would be those with normal auditory system as indicated by their parents. Those who were already diagnosed medically with severe hearing problem were excluded and substituted with other normal respondents.  However, this study did not include otoscopic examination.

Questionnaires interview were administered on all the children to obtain their demographic and socioeconomic information, medical history, daily activities as well as environmental noise exposure from their home and school. Demographic information would include the individual biological data. Socioeconomic include information on the family background, income and parent's education and occupation. Self reported hearing status was also determined from the parents in the questionnaires interview. Medical history covers their previous experience of hearing problems related to otitis media (ear infection) congenital causes such as genetic, period of gestation and diseases suffered by mothers, for example rubella or herpes virus infection, acquired causes such as the delivery difficulties and childhood diseases such as mumps, measles and ototoxic drug or medication prescribed to the children.

For statistical analysis, to determine the variable that influence the hearing thresholds, scores were given to questions related to daily activities, medical history and house environment. Higher scores are given to responses that pose a high risk. Therefore, high total scores means high risk.

The children’s hearing thresholds were assessed by using Diagnostic Audiometer AD 226 conforming to the requirements of the Malaysian Factories and Machineries (Noise Exposure) Regulations of 1989 was used. The test was performed in a sound proof booth and the attenuation of sound in the booths complied with the requirement of the regulation above. The audiometer was calibrated with the same specification daily at the start and end of testing. The air conduction thresholds were measured by a trained personnel for each ear at 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz ⁹.The procedure was explained to the children and the test was performed on the better ear first. Low pure tone average (LPTA) was calculated by averaging air conduction thresholds per ear obtained at 500, 1000, 2000 and 3000 Hz. A high pure-tone average (HPTA) was calculated by averaging air conduction thresholds per ear obtained at frequencies of 4000, 6000 and 8000 Hz. The children was considered as having hearing impairment at low frequency if the average 500, 1000, 2000 and 3000 is greater than 25dB ^10. Hearing impairment at high frequency if the average (4000, 6000, and 8000) is greater than 25dB.

Written consent was obtained from the children's parent for their participation as well as blood collection. Blood was collected through finger prick or capillary method. This method was previously used in Port Pirie, Australia to collect blood from children who live around a lead smelter ¹¹. They found that there was no significant difference between blood lead concentrations obtained through the finger prick and the venous method.

The hand was first soaked in warm water for 10 to 15 minutes to dilate the blood capillaries. To remove contamination, the hand was washed thoroughly with soap and the thumb where the puncture would be made was cleaned with alcohol. Since only 100 ml was needed for direct injection into the Graphite Furnace Atomic Absorption Spectrophotometer (GFAAS) model GBC 908AA made by GBC Scientific Equipment and no extraction was necessary, this method is most convenient and appropriate for small children. In addition, the GFAAS has a detection limit in ppb for blood lead.

The blood samples were diluted 5 times with a matrix modifier solution. Modifications were carried out on the temperature programme of the blood analysis method by ‘GBC Applications’¹² which was then tested against Lyphocheck reference blood lead samples. The Lypocheck test was used as a quality control measures during blood lead analysis. The lyphocheck contains cyclosporin A, red blood folate and blood lead in a known concentration. The blood lead concentration in the lypocheck is in a certain range and after analysis, it gives a reading in a specific standard given. The aim in using lypocheck is to ensure the capability, reliability and accuracy of the instrument in analyzing blood lead when compared to other analysis using the same method.

RESULTS

Demographic and family background of children

The School A sample was made up of 60 boys and 45 girls while the School B sample were made up of 53 boys and 57 girls. All the children were of Malay ethnicity.  The mean number of siblings in the families of the study population was 4. On the average, the children are the third child in their families. Children from School A had bigger family size than the School B children and have lower income.

The mean household income for School B children (RM 2313) was significantly higher than School A children (RM 1448). Table 1 shows that more than 70% families from School A have a total monthly household income below RM 1500, with the highest percentage in the category of RM 500-1000. However, more than 50% families from School B earn above RM 1500, with highest percentage in the category of above RM 2500.

Table 1: Distribution of total household income

Most of the parents have 11 years of formal education followed by 9 years which means that most of them had secondary school education. Parents from School B are more highly educated than School A because many of them attained 13 years or more of formal education. The parents’ occupations have been classified into 7 categories according to Dictionary of Occupational Classification ^13.  Most of the fathers from School A are service workers (23.8%) and professionals (22.9%), while the majority of the fathers from School B are sales personnel (27.3%), followed by professionals (18.2%). On the contrary, the majority of the mothers are housewives.

The majority of the children's house are located on the main roads. The School A children's houses are also located near to construction sites while School B children's houses that are located near an airport or on a highway beside construction sites (Table 2). These sites are sources of environmental noise as well as ambient lead. Most of the children do not listen to radio, walkman or play video game at very high volume (Table 3). Watching television with high volume was the most frequent exposure to noise for both groups of children.

Table 2:  House distance from the sources of noise

Table 3:  Children’s daily activities that produce noise.

According to Table 4, few of the School A children (7.6%) and School B children (10.0%) were born prematurely. The majority of the children did not experience drowning during delivery (93.6- 95.2%). None of the mothers contracted any intrauterine diseases such as rubella, herpes simplex virus infection that can be detrimental to their babies' auditory system. Since the majority of the mothers are house wives, they are not occupationally exposed to any ototoxic chemicals or noise. A small percentage of the children have ear infection (otitis media) (6.7%) and hearing problems (7.3%) during childhood due to diseases such as measles, mumps, chicken pox, meningitis and influenza. A few children from School B had undergone ear operations due to malformation of structures from birth (2.7%). None of the children have taken ototoxic drug.

 Table 4: Information on the Medical History

Hearing Profiles

Table 5 and Table 6 show the children’s hearing threshold classifications based on HPTA and LPTA in both schools. The prevalence of HFHI in School A was lower (12.4%) with 10.5% unilateral hearing impairment and 1.9% bilateral hearing loss when compared to School B (21.7%) with 13.5% unilateral hearing impairment and 8.2% bilateral hearing impairment. About 8.2% of School B children had slight hearing impairment in both ears but only 1.9% School A children had slight hearing impairment in both ears. The majority for both groups of children ( School A: HPTA 87.6%, LPTA 90.5%; School B HPTA 78.3%, LPTA 83.6%) are in the category of bilateral normally hearing (< 25 dB HL).

Table 5: Hearing threshold categories based on high and low pure tone average for School A  children

HT = hearing threshold

Table 6: Hearing thresholds categories based on high and low pure tone average for School B  children

                                            HT = hearing threshold

Blood Lead Concentrations

Blood were sampled from all the children in School A and School B after obtaining written permissions from their parents. The blood lead (PbB) concentrations ranged from 0.35 to 14.72 mg/dl. Only less than 1% of children had PbB level of above 10 mg/dl which is the acceptable standard. ¹⁴

Based on the Kolmogorov Smirnov Normality Test, the frequency distribution that was obtained for blood lead concentration was not normal (p<0.05). The blood lead concentration was transformed to log base 10 and a normal distribution was obtained. Table 7 shows the mean PbB concentration of School A children was 3.44 mg/dl and School B children was 3.16 mg/dl. The difference in term of log to the base PbB was significantly higher for School  A children.

Table 7: Concentrations of blood lead and log₁₀ lead according to schools

Correlation between Hearing Threshold and Blood Lead Concentrations

Table 8 shows the correlation between blood lead concentrations with mean hearing thresholds at high and low frequencies for all children and for each group. There were significant correlation between mean hearing thresholds at high frequencies with PbB in both ears (Right ear: r = 0.192, p = 0.050; Left ear: r = 0.329, p = 0.001) and with thresholds at low frequencies in left ears for (r = 0.273, p = 0.005) only among School A children.

Table 8: Correlation between blood lead with hearing thresholds at HPTA and LPTA for School A  children

Relationship between Hearing Thresholds and Selected Variables

General Linear Model analysis was used to test the significance of the factors that can affect hearing impairment. The results from the analysis show that there was a significant relationship between daily activities scores (F = 4.370, p = 0.038) with right ears hearing thresholds at high frequencies and medical history scores with both ears at low frequencies among all children (Table 9).

Table 9: Relationship between hearing thresholds and selected variables for all children

For School A children, the left ear thresholds at low frequencies was found to have significant relationship with PbB (F = 9.463, p = 0.003) while both ears thresholds at high (Right ear: F = 13.652, p = 0.001: Left ear: F= 14.852,<0.001) and low frequencies (Right ear; F = 15.289, p = 0.001, Left ear, F = 12.914, p = 0.001) have significant relationship with medical history scores (Table 10).  For School B children, the right ( F = 8.641, p= 0.0004) and left ear (F = 3.906, p = 0.049) thresholds at low frequencies were significantly related to the medical history scores (Table 11).

Table 10: Relationship between hearing thresholds and selected variables for School A children

Table 11: Relationship between hearing thresholds and selected variables for School B children

DISCUSSIONS

The socioeconomic status of the School B children is better than the School A children because the family incomes are significantly higher. The mean income of the School B families was slightly higher than the mean monthly gross household income of RM 2162 in 1995 among Malay ethnic in the urban areas^15.  However, the mean income for School A children was lower than RM 2162. TSchool A children are from low class socioeconomic status and the School B children from low middle class socioeconomic status, thus these may be the contributing factors to their poor medical history. However, School B children may be more exposed to noise from their daily activities and home environment as some of their houses are located close to airport or highways beside main roads even though no noise measurements were made. This is supported by the fact that the hearing impairment at high frequencies are higher in School B. However, the study limitation is that no noise measurements were made.

Some of these children are born prematurely, a few experience drowning during birth, have ear operations due to structure malformation during gestation period ear infection due to otitis media and hearing problems as the after effects of certain diseases such as mumps or measles. Inappropriate medical care has probably worsened their hearing threshold which results in hearing impairment. These urban children are at high risk since they are not only exposed to noise but also lead pollution from the motor vehicle gasoline combustion.

The prevalences of HFHI and LFHI in this study are higher than the study in the United States ¹⁶  where 12.7% of US children aged 6 to 19 years had HFHL (9.6% unilateral and 3.1% bilateral) and 7.1% had LFHL (5.6% unilateral and 1.5% bilateral). A study ¹⁷ found that in Malaysia, 76 students (5.81%) failed the screening audiometric test and 95 students (7.26%) had middle ear disorders

Results show a significant correlation between the hearing thresholds at high frequencies in the School A children  at both ears even though the mean PbB concentration in this study was lower than other studies that have been conducted in Malaysia and other countries. Previous study ⁶ found a mean PbB concentrations of 4.30 mg/dl in Malaysia children while in Indonesia the mean PbB concentrations was 7.7 mg/dl ¹⁷. In Mexico, the mean PbB concentrations of 139 children aged 7-9 years was 19.8 mg/dl ¹⁸. This phenomena in Malaysia may be explained by the decreasing atmospheric lead level due to the reduction of lead in gasoline from 0.40 g/L to 0.15 g/L in early 1990¹⁹

The significant correlation between PbB and hearing thresholds among School A children may be explained by the significantly higher PbB in School A children. A study ⁵ found that lead was associated with an increased risk of hearing thresholds that were elevated above the standard reference level at all four frequencies i.e. 500, 1000, 2000 and 4000 Hz. An increase in blood lead, from 6 mg/dl to 18 mg/dl, was associated with a 2 dB loss in hearing at all frequencies. 

The multiple regression model shows that medical history scores were also significantly related to hearing thresholds of both ears at low frequencies among all children. However, the contribution is only about 2% to the right and 0.2% the left hearing thresholds in all the children. Etiology of hearing impairment among 84 Finnish children with unilateral hearing loss was estimated as 2% genetic, 12% congenital non-genetic and 35% in delayed onset non-genetic and 51% remained unknown ²⁰. Slightly more children from the premature group (gestational age £ 35 weeks and birth weight £ 2000 g) with hearing thresholds greater than 15 dB HL. ²¹

The multiple regression model  further confirmed the relationship between blood lead and hearing thresholds at both low and high frequencies in the left ears of School A children. The blood lead contributes to about to 13.7% (LPTA) and 19.1% (HPTA) of the variations in the hearing threshold. Low to moderate lead exposure may affect both central and peripheral segments of the auditory pathway ^{3, 22} . Another study ⁴ revealed that PbB levels were associated with increased right and left hearing thresholds at 500, 1000, 2000 and 4000 Hz.  An individual with PbB level of 20 mg/dl is more likely to have an elevated hearing threshold than an individual with PbB level of 4 mg/dl. In School B children, the hearing thresholds at  low frequencies in both ears are affected by their medical history scores. However, the variations are very small (Right ear: 2%, Left ear; 0.2%). The diseases and infection acquired during childhood and may have affected their auditory system if proper care were not taken during illness.

CONCLUSION

The prevalences of LFHI and HFHI in the School A and B children are low and they made up of a range of 1.9 – 10.9% and 4.6 – 13.5% respectively for both unilateral and bilateral impairment. The hearing impairment is higher in School B than School A children. There was also a significant relationship between medical history scores with both ears thresholds at high and low frequencies in all the studied children.  The relationship between PbB with hearing thresholds at high and low frequencies unilaterally were statistically significant among School A children who have higher PbB concentration. In conclusion, hearing impairment in these children are mainly influenced by their medical history due their poor socioeconomic background, however, lead as well as their daily activities are also contributing to the problem.

ACKNOWLEDGEMENT

Acknowledgement to the Ministry of Education for the permission to carry out the research in the National Primary School. Research funded by the Ministry of Science, Technology and Environment, Malaysia (Grant no. IRPA 06-02-02-0023)

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