Ambient fine particles modify heart rate variability in young healthy adults MAITE VALLEJO, a SILVIA RUIZ, b ANTONIO G. HERMOSILLO, a VI ´ CTOR H. BORJA-ABURTO c AND MANUEL CA ´ RDENAS a a Subdireccio ´n de Investigacio ´n Clı´nica, Instituto Nacional de Cardiologı´a Ignacio Cha ´vez, Tlalpan, Mexico b Departamento de Estadı´stica y Probabilidad, Instituto de Investigacio ´n en Matema ´ticas Aplicadas y en Sistemas, Universidad Nacional Auto ´noma de Me ´xico, Mexico City, Mexico c Coordinacio ´n de Salud en el Trabajo, Instituto Mexicano del Seguro Social, Mexico City, Mexico Particulate air pollution has been related with cardiopulmonary morbidity and mortality. Recent studies have shown that an increase in particulate matter (PM) 2.5 ambient concentrations was associated with a decrease in heart rate variability (HRV) in the elderly with cardiovascular conditions, which could increase the risk of death. In order to assess if this association could also be observed in young adults, we studied 40 young healthy residents of the Mexico City Metropolitan Area (MCMA) who underwent 13 h Holter electrocardiographic and PM 2.5 personal monitoring. HRV was evaluated in time domain: the standard deviation of normal RR intervals (SDNN) and the percentage of differences between adjacent normal RR intervals larger than 50 ms (pNN50). In multivariate analysis with mixed effects models, a significant negative association of pNN50 with PM 2.5 accumulative exposure was found. An increase in 30 mg/m 3 of the average PM 2.5 personal exposure in the previous 2 h decreased the pNN50 in 0.08% (P ¼ 0.01). This observation revealed an acute effect related to environmental exposure to PM 2.5 with regard to HRV in normal youngsters. The long-term health consequences of this association in young healthy adults remain to be clarified. Journal of Exposure Science and Environmental Epidemiology (2006) 16, 125–130. doi:10.1038/sj.jea.7500447; published online 21 September 2005 Keywords: heart rate variability, air pollution, Mexico City, cardiac autonomic function, PM 2.5 . Introduction Air pollution is a major public health problem, which has been associated with an increase in cardiopulmonary morbidity and mortality in many cities around the world after short-term exposures (Dockery and Pope, 1994; Katsouyanni et al., 1995; Wong et al., 2001). Acute as well as chronic exposure to suspended particulate matter (PM) has been linked to a rise in hospital admissions (Zanobetti et al., 2000) and emergency room visits due to respiratory and cardiovascular causes, especially in children under 5 years of age with asthma (Schwartz et al., 1994) and elderly people with known cardiac or pulmonary disease (Linn et al., 2000). Moreover, cohort epidemiological studies have linked long-term exposure to particulate air pollution to a reduction of life expectancy due to cardiovascular mortality (Pope et al., 2004a). Although the underlying biological mechanism of these associations remains limited, several hypotheses have been postulated, from inflammation, accelerated athero- sclerosis and altered cardiac autonomic function. Since the 1980s, heart rate variability (HRV) has been widely used in clinical fields to stratify the risk of arrhythmic death of the patients with ischemic heart disease in whom a low HRV could be a negative predictor (Lombardi et al., 1996). Alterations of the autonomic nervous system accom- panying the early stages of essential hypertension have also studied using HRV and it was found that sympathetic activity increases while parasympathetic decreases. Patients with congestive heart failure had clinical signs of enhanced sympathetic activity and progressive decrease in RR variance (Musialik-Lydka et al., 2003). A low HRV has been associated to an increased mortality rate in people with heart disease (Bigger et al., 1993; Fauchier et al., 2004). Recent studies that included elderly individuals with heart diseases in Baltimore, Boston and Mexico City suggest that PM 2.5 air pollution measured with ambient monitors was associated with a reduced HRV (Liao et al., 1999; Gold et al., 2000; Holguı´n et al., 2003; Park et al., 2005). The aim of this study was to test whether this association could also be observed in young healthy adults, using personal exposure monitors, at ambient levels currently observed in the Mexico City metropolitan area (MCMA). Received 4 April 2005; accepted 10 June 2005; published online 21 September 2005 1. Address all correspondence to: Dr M Vallejo, Direccio´ n de Investigacio´ n 71 piso, Instituto Nacional de Cardiologı´a, Ignacio Cha´ vez, Juan Badiano No. 1. Col. Seccio´ n XVI, Tlalpan 14080, Me´ xico. Tel.: þ 52 55 55 73 29 11 exts. 1223 and 1357. Fax: þ 52 55 55 73 09 26. E-mail: maite_vallejo@yahoo.com.mx Journal of Exposure Science and Environmental Epidemiology (2006) 16, 125–130 r 2006 Nature Publishing Group All rights reserved 1559-0631/06/$30.00 www.nature.com/jes