| Summary: | The North Atlantic Oscillation (NAO) is the most prominent and recurrent pattern of atmospheric variability over the middle and high latitudes of the Northern Hemisphere, especially during the cold season months. It is possible that variations in NAO indices (NAOI) had additional impact on human health, after adjusting for weather variables and air pollutants. We investigated the association between daily emergency ambulance calls (EACs) for elevated blood pressure during whole day and during the time intervals of 8:00–13:59, 14:00–21:59, and 22:00–7:59 and daily NAO indices lower than 20th percentile (-1.0) and higher than 85th percentile (0.5) (study period January 1, 2009 – June 30, 2011). We used multivariate Poisson regression, adjusting for seasonality, day length, the day of the week, air temperature, atmospheric pressure, wind speed, exposure to CO, PM10, and ozone, active-stormy geomagnetic activity, and high-speed solar wind. Rate ratios (RRs) with 95% confidence interval were used to assess the risk of EACs on days of NAOI < -1 and NAOI >0.5 as compared to other days. During the period of the study, were used 17,114 cards of EACs at Kaunas city, Lithuania ambulance service. NAOI < -1 and NAOI >0.5 was associated an increased the risk of daily EACs: in all subjects, RRs were, respectively, 1.05 (1.01-1.10) and 1.04 (0.99-1.09), in the elderly (age >65 years) these RRs were 1.06 (1.01-1.12) and 0.99 (0.92-1.05), and in younger subjects, respectively, 1.03 (0.97-1.11) and 1.13 (1.04-1.22). The stronger impact of NAOI during November-March was observed in all and in younger subjects: the RRs of EACs for daily NAOI <-1 and NAOI >0.5 were, respectively, 1.07 (1.01-1.15) and 1.09 (1.00-1.18) in all subject and 1.13 (1.02-1.26) and 1.16 (1.02-1.32) in the younger. During November-March, an increase the risk of EACs in the elderly was observed on days of NAOI>0.5 only during 14:00-21:59 (RR=1.14 (1.01-1.30
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