Table 2.

Meta-analysis results of amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases (NDD)

		Fixed effects model			Random effects modela			Heterogeneity
Analysis	N	Meta-RR	CIL	CIU	Meta-RR	CIL	CIU	X2	P
Amyotrophic lateral sclerosis (ALS)
Main-highest peak exposure	6	1.37	1.21	1.56	1.78	1.20	2.65	19.08	< 0.01
Ever exposureb	6	1.25	1.17	1.33	1.17	0.93	1.47	20.59	< 0.01
Including studies with proxy exposurec	11	1.10	0.98	1.23	1.18	0.65	2.17	158.33	< 0.01
Exclude one ALS studyd
Fang et al. (2009)	5	1.36	1.20	1.55	1.73	1.14	2.60	17.96	< 0.01
Weisskopf et al. (2009)	5	1.30	1.14	1.48	1.31	1.08	1.59	4.96	0.29
Pinkerton et al. (2013)	5	1.37	1.21	1.56	1.89	1.24	2.88	18.77	< 0.01
Roberts et al. (2016)	5	1.36	1.19	1.54	1.65	1.12	2.45	16.10	< 0.01
Peters et al. (2017)	5	1.43	1.23	1.65	2.24	1.09	4.61	17.98	< 0.01
Seals et al. (2017)	5	1.52	1.22	1.90	2.21	1.05	4.66	17.75	< 0.01
Other NDD subtypes
Parkinson’s diseaseb	9	1.23	1.10	1.38	1.70	1.17	2.48	46.49	< 0.01
Multiple sclerosisb	5	0.54	0.43	0.68	0.80	0.37	1.73	35.33	< 0.01
Total NDD in EE studies
Plastics	5	4.06	2.63	6.25	3.64	1.84	7.19	7.44	0.11
Wood workersb	8	1.19	1.02	1.38	1.10	0.86	1.42	9.35	0.23
Textile workersb	12	0.73	0.65	0.83	0.77	0.44	1.34	158.22	< 0.01

CI confidence interval, EE exposure estimated, NDD neurodegenerative disease, N number of studies, meta-RR meta-analysis relative risk

^aRandom effects model was used when X² heterogeneity statistic > degrees of freedom (number of studies minus 1)

^bNot reported in Roberts et al. 2016 and was calculated by the authors

^cIncludes additional papers where formaldehyde exposure was not clearly defined but included professions with known exposure (plastics manufacturing, woodwork, and textile work)

^dOne study excluded at a time.