Table 1 Descriptive summaries for all included studies

Pan L et al. (2018) [24]

Beijing (China); March 28th to May 21th, 2016.

8 elderly subjects with COPD (5 males, 3 female) and 8 Healthy spouses (3 males, 5 female),73 yr.

SDNN, rMSSD, HF, LF

Individual monitor; 12-h average; NA

48.4 ± 45.3

Shutt RH et al. (2017) [25]

Sault Ste (Canada), 5 consecutive 8-h days outdoors in one of two locations (Bayview and College sites). Urban. NA

60 healthy adults (28 males, 32 female), 24.2 yr.

SDNN, rMSSD, LF, HF

Fixed site monitors; 8-h average (averaged over the 6-h period before the visit and 2-h after the visit); NA

College: 11.67 ± 6.61

Bayview: 13.01 ± 6.87

Lim YH et al. (2017) [26]

Seoul (Korea), three health examinations between 2008 and 2010.

466 elderly subjects (111 males, 355 female), 70.6 yr.

SDNN, rMSSD, HF, LF

Fixed monitoring station located on the roof of a building; 24-h average; lag0^#, lag01^#, lag02^#, lag03^#, lag04^#, lag05^#, lag06^#, lag07^#, lag08^#, lag 09 days^#.

27.0 ± 13.3

Chen SY et al. (2017 )[21]

Taiwan (China), during December 2002 through September 2003.

61 patients with multiple CVD risk factors (33 males, 28 female), 62.9 yr.

SDNN, rMSSD, HF, LF

Fixed air quality monitoring station, Sinjhuang Supersite; 24-h average (1-day lag); lag1 day, lag3 day.

41.4 ± 21.6

Lee MS et al. (2016) [27]

Boston (USA), March 1, 2004, to August 31, 2004.

21 adults without heart disease (4 males, 17 female), 44yr, 19% male.

SDNN

Individual monitor; 5-min average; lag0h-lag4h^#.

29.8 ± 77.7

Xie Y et al. (2016) [28]

Shanghai (China). September to December 2013.

619 men and women aged from 35–75 yr (235 males, 384 female), 56.7 yr, 38% male.

HF, LF

Individual monitor; 10-h average; NA

90.2 ± 61.3

Peters A et al. (2015) [29]

Germany, March 19, 2007 and December 17, 2008.

64 individuals with type 2 diabetes and impaired glucose tolerance, 66 yr, NA.

SDNN, rMSSD

Central monitoring site; 1-h average; NA

13.7 ± 11.2

Liu WT et al. (2015) [30]

Taiwan (China), between January and March in the years 2012 to 2014.

120 young, healthy subjects (58 males, 62 female), 21.2 yr.

SDNN, rMSSD

Individual monitor; 5-min average; NA

22.3 ± 6.9

Lee MS et al. (2014) [15]

Boston (USA), March to August 2004.

21 community members, 44 yr (4 males, 17 female), NA.

SDNN

Individual monitoring; 5-min average; NA.

29.8 ± 77.7

Xu MM et al. (2013) [31]

Beijing (China), July 2007–September, 2008.

42 patients with heart disease (14 males, 28 female), 54–78 yr.

SDNN, rMSSD, HF, LF

Fixed site monitors; 20-h average; lag0, lag1 (21–40 h), lag2 (41–60 h).

80.6 ± 35.0

Bartell SM et al. (2013) [5]

Los Angeles (USA), 2005 to 2007.

50 participants from four retirement communities with coronary artery disease (31 males, 19 female), ≥ 71 yr.

SDNN, rMSSD

Individual monitor; 1 h^#, 4 h^#, 8 h^#, 24 h average, 3 days^#, 5 days^#; NA.

21.1 ± 11.4

Shields KN et al. (2013) [32]

Mexico City Metropolitan Area (Mexico), February 11 to 23, 2002.

16 researchers (11 males, 5 female), 22–56 yr,.

SDNN, HF, LF

Van-based mobile laboratory: 5-min average, 30-, 60-, and 90-min; NA.

14 ± 8

Huang J et al. (2013) [33]

Beijing (China), May 2011 to October 2011.

40 young healthy adults (16 males, 24 female), 24.4 yr.

SDNN, rMSSD, HF, LF

Individual monitor; 5-min average, 15 min^#, 30 min^#, 1 h^#; NA.

Median:

Traffic center:162.10 Park:53.00

Jia X et al. (2012) [34]

Beijing (China), August 2008 to September 2008.

30 healthy elderly subjects (12 males, 18 female), 57.9 yr.

HF, LF

Fixed monitors on a six-floor rooftop; 5-min average, 15 min^# 30 min^#, 1 h^#, 2 h^#, 4 h^# ,6 h^#; NA.

median: 44.09

Rich DQ et al. (2012) [35]

New York state (USA), June 2006 to November, 2009.

76 participants with MI or unstable angina (51 males, 25 female), NA, 67% male.

SDNN, rMSSD

Fixed site monitors: a wide range particle spectrometer; 31-h average (averaged over the 24-h period before the visit as well as a shorter lag period (lag 0–5 h)); lag h 24–47 h^#, 48–71 h^#, 72–95 h^#, 96–119 h^#.

8.67 ± 6.06

Hampel R et al. (2012) [36]

Augsburg (Germany), March 2007 to December, 2008.

61 subjects with diabetes or impaired glucose tolerance (40 males, 21 female), 67.5 yr.

SDNN, rMSSD

Fixed site monitors in Augsburg; 1-h averages; lag1h, lag2h lag3h, lag4h, lag 5h, lag 6h.

13.7 ± 11.2

Huang W et al. (2012) [37]

Beijing (China), during summer 2007 and summer 2008.

40 subjects with CVD (16 males, 24 female), 65.6 yr.

SDNN, rMSSD, HF, LF

Air monitoring station; 1 h^#, 4 h average, 12 h^#; NA

Visit 1: 112.5 ± 61.3

Visit 2: 78.3 ± 50.6

Visit 3: 89.2 ± 53.9

Visit 4: 64.2 ± 39.9

Wu CF et al. (2010) [14]

Taiwan (China), February to March 2007.

17 mail carriers (17 males, 0 female), 32.4 yr, 100% male.

SDNN, rMSSD, HF, LF

Individual monitor; 7-h average; NA.

68.2 ± 30.0

Wu S et al. (2010) [17]

Beijing (China), May 2008-March 2009.

9 healthy taxi drivers (4 males, 5 female), 27–39 yr.

SDNN, rMSSD, HF, LF

Individual monitor;30-mim average, 2-h average; NA

Before: 95.4 ± 58.6;

During: 39.5 ± 25.2;

After: 64.0 ± 80.3

Chuang KJ et al. (2007) [38]

Taiwan (China), April-June of 2004 or 2005.

76 healthy college students (46 males, 30 female), 21 yr.

SDNN, rMSSD, HF, LF

Fixed site: 1-day average, 2-day, 3-day; NA.

31.8 ± 10.6

Zanobetti A et al. (2010) [39]

Boston (USA), 1999 to 2003.

46 patients with coronary artery disease (37 males, 19 female), 43–75 yr.

SDNN, rMSSD, HF

Fixed site monitors: 30 min^#, 1 h^#, 2 h average, 48 h^#, 72 h^#, 96 h^# ,120 h^#; NA.

Median: 9.54

Suh HH et al. (2010) [16]

Atlanta (USA), Fall 1999 and Spring 2000.

30 subjects: 12 with a recent MI and 18 with COPD (17 males, 13 female), 65 yr.

SDNN, rMSSD, HF, LF

Individual monitor; 24-h Ambient, 24h personal; NA.

15.78 ± 8.75

Schneider A et al. (2010) [40]

Erfurt (Germany), October 2000 and April 2001.

56 patients with ischemic heart disease, stable angina pectoris or prior myocardial infarction at an age of at least 50 years (56 males, 0 female). 66 yr.

SDNN, rMSSD, HF, LF

Fixed monitoring site: 24-h average; NA

20.3 ± 14.8

Folino AF et al. (2009 )[41]

Padua (Italy), Summer 2006, Winter and Spring 2007.

39 patients with MI (36 males, 3 female),45–64 yr, 92% male

SDNN, rMSSD

Individual monitor: 24-h average; NA

Visit 1: 33.9 ± 12.7;

Visit 2: 62.1 ± 27.9;

Visit 3: 30.8 ± 14.0

Yeatts K et al. (2007) [42]

North Carolina (USA), a 12-week period, September 2003 to July 2004.

12 adult asthmatics (3 males, 8 female), 33 yr,.

SDNN, rMSSD

Fixed site monitors:24-h average; NA

12.5 ± 6.0

Adar SD et al. (2007) [43]

Missouri (USA), March and June of 2002.

44 nonsmoking seniors (7 males, 37 female), 62–94 yr, 16% male.

SDNN, rMSSD, HF, LF

Individual monitor: 5-min^#, 30-min^#, 1-h^#, 4-h^#, and 24-h average; NA.

7.7 ± 6.8

Luttmann-Gibson H et al. (2006) 40]

Ohio (USA), during summer (June 4 to August 18) and fall (September 25 to December 15) of 2000.

32 nonsmoking senior adults (3 males, 19 female), 70.8 yr, 9% male.

SDNN, rMSSD, HF, LF

Ambient monitoring site at located at the Franciscan University: 24-h average; NA.

19.7

Wheeler A et al. (2006) [18]

Atlanta (USA), In fall 1999 and spring 2000.

18 individuals with COPD and 12 individuals with MI (17 males, 13 female), 65 yr, 57% male.

SDNN, rMSSD, HF, LF

Fixed site monitors; 4-h average; NA.

17.8

Timonen KL et al. (2006) [44]

Erfurt (Germany), Helsinki (Finland), Amsterdam (Netherlands),1998 to 1999.

131 subjects with stable coronary artery disease (90 males, 41 female), 40–84 yr.

SDNN, HF

Fixed site monitors; 24-h average, 5-day average; lag0, lag1, lag2, lag3 day.

Amsterdam: 20.0;

Erfurt: 23.1;

Helsinki:12.7

Riediker M et al. (2004) [45]

North Carolina (USA), Fall 2001.

9 healthy policemen (9 males, 0 female),23–30 yr.

SDNN, HF, LF

Individual monitor; 24-h average; NA.

23

Schwartz J et al. (2005) [46]

Massachusetts (USA), 12 weeks during the summer of 1999.

28 elderly subjects aged 61–89 yr (7 males, 14 female), 71 yr.

SDNN, rMSSD

Fixed monitoring site at Harvard University: 1-h average; NA.

Median: 10

Magari SR et al. (2002) [47]

USA, June to December 1999.

20 relatively young, healthy male workers (20 males, 0 female), 43 yr, 100% male.

SDNN

Individual monitoring: 3-h average; NA

96 ± 158

Brauer M et al. (2001) [48]

Vancouver (Canada), April-September. 1998.

16 patients with COPD (7 males, 9 female), 54–86 yr.

SDNN, rMSSD

Individual monitor; 24-h average; NA.

18.2 ±14.6