Table 1

Characteristics of the studies included in the meta-analysis

Author	Year	Population	Number of persons	Age	Men, %	Study design (measurements per person)	Exposure	Scale of HRV	Length of analysed ECG recordings	Average PM concentration, μg/m³
Liao12	1999	All‖	4899	62.4±5.7^B	43	Cross-sectional	24 h PM_2.5	HF and LF Log	5 min	16.99±8.04^B^*
		Without hypertension	8	78 (65–84)^A	37
		With hypertension	18	82 (69–89)^A	22
Pope13	1999	With CVD/respiratory disease	7	75 (23–82)^C	85.7	Longitudinal (4.1)	24 h PM₁₀	Lin	24 h	83.92^D^*
Gold14	2000	Elderly	21	73.3 (53–87)^C	48	Longitudinal (7.76)	4 h PM_2.5	Lin	5 min	15.3 (2.9–48.6)^A
Brauer15	2001	With COPD	16	74 (54–86)^A	43.8	Longitudinal (7)	24 h PM_2.5	Lin	24 h	10.8±1.4^E
Magari16	2001	Occupational	33	38.1±12.9^B	100	Longitudinal (2)	4 h PM_2.5	Log	24 h	223±2203^E
Holguin17	2003	Without hypertension	21	80 (65–96)^A	52	Longitudinal (18)	24 h PM_2.5	Log	5 min	30.4±9.9^B
Holguin17	2003	With hypertension	13	78 (60–88)^A	30	Longitudinal (18)	24 h PM_2.5	Log	5 min	30.4±9.9^B
Liao18	2004	All Without hypertension	4899	62.4±5.7^B	43	Cross-sectional	24 h PM₁₀	HF and LF Log	5 min	16.99±8.04^B^*
Liao18	2004	With hypertension	4899	62.4±5.7^B	43	Cross-sectional	24 h PM₁₀	HF and LF Log	5 min	16.99±8.04^B^*
Pope19	2004	Elderly	88	54–89^F	43	Longitudinal (2.84)	24 h PM_2.5	Log	24 h	23.7±20.2^B
Riediker20	2004	Occupational, young	9	27.3 (23–30)^A	100	Longitudinal (4)	9 h PM_2.5	Lin	10 min	23 (7.1–38.7)^A^‡
Park21	2005	Elderly (NAS)	497	72.7±6.6^B	100	Cross-sectional	48 h PM_2.5	Log	4 min	11.4±8.08^B
Schwartz22	2005	Elderly	27	74 (64–81)^G	Predominately female	Longitudinal (12)	24 h PM_2.5	Log	30 min	10 (7–17)^G
Sullivan23	2005	With CVD	21	77 (57–85)^C	43	Longitudinal (8.38)	24 h PM_2.5	Ln	20 min	10.7 (8.0–15.6)^G
Sullivan23	2005	Without CVD	13	78 (72–87)^C	46	Longitudinal (8.38)	24 h PM_2.5	Ln	20 min	10.7 (8.0–15.6)^G
Lipsett6	2006	With CAD	19	71.3±6.0^B	63.2	Longitudinal (12)	6 h PM₁₀	Log	24 h
Luttmann-Gibson24	2006	General	32	70.8 (54–90)^A	9	Longitudinal (21.8)	24 h PM_2.5	Log	30 min	19.7 (11.6–25.0)^H
Timonen25	2006	With CAD, Amsterdam	37	71.5±8.3^B	65	Longitudinal (10.92)	72 h PM_2.5	HF Ln	25 min	20.0 (10.4–23.9)^H
		With CAD, Ertfurt	47	64.6±8^B	91			SDNN, rMSSD		23.1 (10.5–27.4)^H
		With CAD, Helsinki	47	68.3±6.4^B	55			Log		12.7 (8.1–16.0)^H
Vallejo26	2006	Young	40	27 (21–35)^A	27.5	Longitudinal (33)	3.5 h PM_2.5	Ln	13 h	74 (49–111)^G
Riojas-Rodriuguez 27	2006	General	30	55 (25–76)^A	83	Longitudinal (111)	11 h PM_2.5	Ln	11 h	46.8±1.82^E
Chuang28	2007	Young	76	20.8±1.2^B	66.7	Longitudinal (3)	24 h PM_2.5	Log	16 min	31.8±10.6^B
Chuang29	2007	Elderly	46	70.0±11.7^B	41	Longitudinal (168)	4 h PM_2.5	Log	24 h	51.7±40.1^B
Min30	2008	General	1349	44.3±21.9^B	43.63	Cross-sectional	24 h PM₁₀	Log	5 min	23.25±13.3^B^*
Folino31	2009	With MI	39	60.3±4.8^B	92.3	Longitudinal (2.7)	24 h PM_2.5	Lin	24 h	33.9±12.7^B
Whitsel32	2009	All	5470	64±0.1^B^†	0	Longitudinal (3)	24 h PM₁₀	Log	10 s	19.6±0.14^B^*
		Diabetes	770
		Impaired fasting glucose	1559
		Normal fasting glucose	3141
Zanobetti33	2009	With CAD	46	57 (43–75)^C	80.4	Longitudinal (4)	2 h PM_2.5	Log	24 h	8.92 (5.65–13.58)^G
Park34	2010	General	5465	62.2±10.9^B	46.8	Cross-sectional	24 h PM_2.5	Log	30 s	14.3 (10.2–20.4)^G
Schneider35	2010	With CAD	56	66±6^B	100	Longitudinal (12)	24 h PM_2.5	Log	24 h	20.3±14.8^B
Suh36	2010	All	30	65 (55–73)^H		Longitudinal (7)	24 h PM_2.5	Log	25 min
		With COPD	18		38.9					16.54±9.33^B
		With MI	12		83.3					14.65±7.7^B
Wu37	2010	Occupational	11	35.5 (27–41)^A	45	Longitudinal (3)	2 h PM_2.5	Log	12 h	56.6 (34.6–104.1)^G
He38	2011	General	106	56.23±7.61^B	40.57	Longitudinal (48)	6 h PM_2.5	HF,LF Log	24 h	13.21±14.44^B
He38	2011	General	106	56.23±7.61^B	40.57	Longitudinal (48)	6 h PM_2.5	rMSSD, SDNN Lin	24 h	13.21±14.44^B
Jia39	2011	General	30	57.9±5.4^B	40	Longitudinal (116)	24 h PM_2.5	Log	24 h	37.52(0.19–214.5)^I

Descriptive statistics were only given for all participants. No calculations could be made for rMSSD.
Population: description of persons included.
Age, Average PM concentration: A, arithmetic mean (range); B, arithmetic mean ± SD; C, median (range); D, mean; E, geometric mean ± SD; F, range; G, median (25%–75% percentile); H, arithmetic mean (25%–75% percentile); I, median (1%–99% percentile).
↵* Average PM_2.5 concentration calculated with the assumption that PM₁₀ consists of 70% of PM_2.5.
↵† Average age/PM_2.5 concentration during the first visit.
↵‡ Mass, after shift.
CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; CVD, cardiovascular disease; MI, myocardial infarction; NAS, Normative Aging Study; PM_2.5, particulate matter with aerodynamic diameter of 2.5 μm or less; PM₁₀, particulate matter with aerodynamic diameter of 10 μm or less. ‖Descriptive statistics were only given for all participants. No calculations could be made for rMSSD.