Respirable dust and crystalline silica concentrations among workers at a brick kiln in Bhaktapur, Nepal

Figures & data

Table 1. Associations between job category and personal breathing zone respirable dust concentrations^a among workers^b at a brick kiln in Bhaktapur, Nepal, March 2023.

	Respirable dust (mg/m^3)
		MI Sample			MI TWA
Characteristic	Above LDL^c, n (%)	GM	95% CI	p-value	GM	95% CI	p-value
Overall	49 (100)	0.20^d	0.16, 0.27^d		0.12^d	0.09, 0.16^d
Job category
Administration	7 (14)	0.11^e	0.08, 0.15^e		0.07^e	0.04, 0.10^e
Fire master	8 (16)	0.14^e	0.10, 0.19^e		0.06^e	0.04, 0.09^e
Green brick hand molder	16 (33)	0.15^e	0.12, 0.19^e		0.07^e	0.05, 0.09^e
Green brick machine molder	11 (22)	0.18^e	0.14, 0.24^e		0.14^e	0.09, 0.18^e
Top loader	7 (14)	1.49^e	1.08, 2.07^e	<0.0001^e^,^f	0.99^e	0.65, 1.50^e	<0.0001^e^,^g

Abbreviations: CI, confidence interval; GM, geometric mean; LDL, lower detection limit; MI, multiply imputed; OSHA, U.S. Occupational Safety and Health Administration; PEL, permissible exposure limit; TWA, eight-hour time-weighted average.

^a The OSHA PEL for respirable dust is a TWA of 5 mg/m³ (Occupational Safety and Health Administration 2023b).

^b One sample was missing because one of the 50 workers enrolled in the study declined to participate in the personal breathing zone air sampling.

^c The LDL mass for respirable dust was 0.02 mg.

^d Estimated using multiple imputation and intercept only linear regression models of the natural logarithm transformed values.

^e Estimated using multiple imputation and simple (i.e., unadjusted) linear regression models of the natural logarithm transformed values.

^f After adjusting for multiple comparisons using the Tukey-Kramer method, tests of pairwise differences among job categories had the following p-values: Administration vs. Fire master: 0.82, Administration vs. Green brick hand molder: 0.50, Administration vs. Green brick machine molder: 0.13, Administration vs. Top loader: <0.0001, Fire master vs. Green brick hand molder: 0.99, Fire master vs. Green brick machine molder: 0.68, Fire master vs. Top loader: <0.0001, Green brick hand molder vs. Green brick machine molder: 0.79, Green brick hand molder vs. Top loader: <0.0001, Green brick machine molder vs. Top loader: <0.0001.

^g After adjusting for multiple comparisons using the Tukey-Kramer method, tests of pairwise differences among job categories had the following p-values: Administration vs. Fire master: >0.99, Administration vs. Green brick hand molder: >0.99, Administration vs. Green brick machine molder: 0.13, Administration vs. Top loader: <0.0001, Fire master vs. Green brick hand molder: >0.99, Fire master vs. Green brick machine molder: 0.07, Fire master vs. Top loader: <0.0001, Green brick hand molder vs. Green brick machine molder: 0.05, Green brick hand molder vs. Top loader: <0.0001, Green brick machine molder vs. Top loader: <0.0001.

Table 2. Associations between job category and personal breathing zone respirable crystalline SiO₂ concentrations^a among workers^b at a brick kiln in Bhaktapur, Nepal, March 2023.

			MI Sample			MI TWA
SiO2 Type/Characteristic	Below LDL^c, n (%)	Above LDL^c, n (%)	GM	95% CI	p-value	GM	95% CI	p-value
Cristobalite, µg/m^3
Overall	49 (100)	0 (0)
Quartz, µg/m^3
Overall	27 (55)	22 (45)	15.28^d	11.11, 21.02^d		8.60^d	5.99, 12.34^d
Job category
Administration	4 (15)	3 (14)	8.81^e	5.99, 12.97^e		5.49^e	3.36, 8.98^e
Fire master	8 (30)	0 (0)	7.82^e	5.43, 11.25^e		3.62^e	2.28, 5.73^e
Green brick hand molder	12 (44)	4 (18)	10.06^e	7.72, 13.11^e		4.69^e	3.37, 6.54^e
Green brick machine molder	3 (11)	8 (36)	13.84^e	10.19, 18.81^e		9.97^e	6.75, 14.73^e
Top loader	0 (0)	7 (32)	173.08^e	117.96, 253.95^e	<0.0001^e^,f	114.39^e	70.19, 186.43^e	<0.0001^e^,g
Tridymite, µg/m^3
Overall	49 (100)	0 (0)

Abbreviations: CI, confidence interval; GM, geometric mean; LDL, lower detection limit; MI, multiply imputed; OSHA, U.S. Occupational Safety and Health Administration; PEL, permissible exposure limit; SiO₂, silica; TWA, eight-hour time-weighted average.

^a The OSHA PEL for respirable crystalline SiO₂ is a TWA of 50 µg/m³ (Occupational Safety and Health Administration 2022).

^b One sample was missing because one of the 50 workers enrolled in the study declined to participate in the personal breathing zone air sampling.

^c The LDL masses for respirable crystalline cristobalite, quartz, and tridymite were 5, 5, and 20 µg, respectively.

^d Estimated using multiple imputation and intercept only linear regression models of the natural logarithm transformed values.

^e Estimated using multiple imputation and simple (i.e., unadjusted) linear regression models of the natural logarithm transformed values.

^f After adjusting for multiple comparisons using the Tukey-Kramer method, tests of pairwise differences among job categories had the following p-values: Administration vs. Fire master: 0.99, Administration vs. Green brick hand molder: 0.97, Administration vs. Green brick machine molder: 0.39, Administration vs. Top loader: <0.0001, Fire master vs. Green brick hand molder: 0.78, Fire master vs. Green brick machine molder: 0.15, Fire master vs. Top loader: <0.0001, Green brick hand molder vs. Green brick machine molder: 0.52, Green brick hand molder vs. Top loader: <0.0001, Green brick machine molder vs. Top loader: <0.0001.

^g After adjusting for multiple comparisons using the Tukey-Kramer method, tests of pairwise differences among job categories had the following p-values: Administration vs. Fire master: 0.73, Administration vs. Green brick hand molder: 0.98, Administration vs. Green brick machine molder: 0.35, Administration vs. Top loader: <0.0001, Fire master vs. Green brick hand molder: 0.88, Fire master vs. Green brick machine molder: 0.02, Fire master vs. Top loader: <0.0001, Green brick hand molder vs. Green brick machine molder: 0.04, Green brick hand molder vs. Top loader: <0.0001, Green brick machine molder vs. Top loader: <0.0001.

Occupational Safety and Health Administration. 2023b. Particulates not otherwise regulated, total and respirable dust (PNOR). Washington (DC): U.S. Department of Labor, Occupational Safety and Health Administration; [accessed 2023 Aug 7]. https://www.osha.gov/chemicaldata/801.

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Occupational Safety and Health Administration. 2022. Silica, crystalline. Washington (DC): U.S. Department of Labor, Occupational Safety and Health Administration; [accessed 2022 Jul 26]. https://www.osha.gov/silica-crystalline.

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Data availability

The data that support the findings of this study are available from the corresponding author, JDB, upon reasonable request.