**Jone Vencloviene 1,\*, Agne Braziene 1, Jurate Zaltauskaite <sup>1</sup> and Paulius Dobozinskas <sup>2</sup>**


Received: 31 May 2018; Accepted: 26 July 2018; Published: 28 July 2018

**Abstract:** The North Atlantic Oscillation (NAO) is the most prominent pattern of atmospheric variability over the middle and high latitudes of the Northern Hemisphere, especially during the cold season. It is found that "weather types" are associated with human health. It is possible that variations in NAO indices (NAOI) had additional impact on human health. We investigated the association between daily emergency ambulance calls (EACs) for exacerbation of essential hypertension and the NAOI by using Poisson regression, adjusting for season, weather variables and exposure to CO, particulate matter and ozone. An increased risk of EACs was associated with NAOI < −0.5 (Rate Ratio (RR) = 1.07, *p* = 0.013) and NAOI > 0.5 (RR = 1.06, *p* = 0.004) with a lag of 2 days as compared to −0.5 ≤ NAOI ≤ 0.5. The impact of NAOI > 0.5 was stronger during November-March (RR = 1.10, lag = 0, *p* = 0.026). No significant associations were found between the NAOI and EACs during 8:00–13:59. An elevated risk was associated during 14:00–21:59 with NAOI < −0.5 (RR = 1.09, *p* = 0.003) and NAOI > 0.5 (RR = 1.09, *p* = 0.019) and during 22:00–7:59 with NAOI < −0.5 (RR = 1.12, lag = 1, *p* = 0.001). The non-linear associations were found between the NAO and EACs. The different impact of the NAO was found during the periods November–March and April–October. The impact of the NAOI was not identical for different times of the day.

**Keywords:** North Atlantic Oscillation; weather; emergency ambulance calls; exacerbation of essential hypertension
