Wildfires have a considerable impact on the environment and humans worldwide. Climate change has lately been identified as a very important variable in this matter since the future projections suggest an increase in the number of droughts, heat waves and dry spells . Global warming will produce changes in temperature and precipitation patterns leading to a higher prevalence and severity of wildfires and consequently impacting future air quality . In fact, this could not only extend the burned area in chronically impacted areas but also affect new ones, like Sweden in the summer of 2018 . According to the 2016 European Forest Fire Information System (EFFIS) report , the south of Europe (Portugal, Spain, France, Italy and Greece) is the area most affected by wildfires from 1980 until today, considering Europe, the Middle East and North Africa. In the last decades, Portugal was by far the country with the largest burned area, almost 50 %, between the southern European countries . Although there has been a slight decreasing trend in the burned area in this region since 2000 after an increasing period in the previous 20 years (European Environment Agency, https://www.eea.europa.eu/data-and-maps/indicators/forest-fire-danger-3/assessment, last access: 30 June 2020), recent extreme events like the 2017 fires in Portugal and the 2018 fires in Greece which resulted in a severe loss of human lives are confirming the worst-case projections.

Uncontrolled wildfires emit numerous pollutants derived from the incomplete combustion of biomass fuel which cause damage to human health, particularly the cardiovascular and respiratory systems . Examples include particulate matter (PM), carbon monoxide, methane, nitrous oxide, nitrogen oxides, volatile organic compounds (VOCs) and other secondary pollutants that are released mainly into the atmosphere but can be transported to many other environmental compartments. Moreover, they can affect the physicochemical properties of the atmosphere, like, for instance, the interaction of PM with solar radiation which can prompt a modification of the temperature depending on the characteristics of the aerosol . Consequently, some of these chemicals are regulated by the European Directive 2008/50/EC of 21 May 2008 of the European Parliament and of the Council on Ambient Air Quality and Cleaner Air for Europe, which establishes threshold values for a safe air quality. But although wildfire emissions are a crucial parameter for the local air quality , where in some cases there are already chronically exposed populations due to the frequency and dimension of the events, they are not contained by political borders and can also affect areas far from the ignition points due to the atmospheric transport of the pollutant plumes. A number of studies (e.g. , among others) report the influence of natural and anthropogenic emissions on air quality composition across different countries, especially PM and tropospheric O3. For wildfires it is also important to take into account some factors which influence the plume dispersion, such as the duration and space evolution of the fire event and the meteorological conditions associated with it . An increase in cardiovascular and respiratory morbidity and mortality are some of the impacts these contaminants can have on humans . For instance, there is strong evidence for the relationship between PM in general and mortality, especially from cardiovascular diseases, for both long-term and short-term exposure . Although some studies corroborate the existence of a link between the exposure to wildfire-related air pollutants and hospital admissions, visits to emergency clinics or even respiratory morbidity , the impacts on human health are difficult to quantify and the real effects still poorly known.

Concerning PM, a recent study focusing on 10 southern European cities revealed that cardiovascular and respiratory mortality associated with PM10 (particles with an aerodynamic diameter below 10 µm) was higher on days affected by wildfires' smoke than in smoke-free days . The authors also found that PM10 from forest fires increased mortality more than PM10 from other sources. So, the estimation of mortality due to exposure to wildfire-generated pollutants is key to manage health resources and the necessary public funds towards prevention and remediation in setting up appropriate policies and protocols .

The two main factors to take into account for the wildfire's effects are the location and, most importantly, the size of the fire event (characterised by the respective burned area). When the wildfire occurs close to a large conurbation, the population exposed is higher. But as showed in their study, small fires do not seem to have an effect on mortality, whereas medium and large episodes (with burned areas > 1000 ha) have a significant impact on human health, which increases with the size of the fire. Aiming to enhance the knowledge on the effects of wildfires on human health, this study describes the pattern of wildfires in Portugal for 16 years (2001–2016) and assesses the impact of those events on the country's population mortality during the fire season (June, July, August and September). In this work, the focus is placed on indirect effects of pollutants emitted by wildfires, namely assessing the influence of wildfire-generated PM10 on the Portuguese population mortality. The relationship between the burned area of large wildfires and PM10 and this same pollutant and mortality was studied. The Nomenclature of Territorial Units for Statistics (NUTS) level 3 (NUTS III) geographical division has been used to be able to compare the effects of the fires in different parts of the country. Finally, monthly deaths due to all-cause (excluding injuries, poisoning and external causes) and cause-specific mortality (cardiovascular and respiratory) for all ages for each NUTS III region have been studied. These causes have been selected due to their well-known connection with air pollution.

In this study, the effects of short-term exposure to pollutants due to wildfires on human mortality were quantified. The forest fire pollutant emissions were estimated for the period 2001–2016 during the summer months (June–July–August–September) in mainland Portugal (23 NUTS III regions and more than 10 million people). In Portugal, large forest fires usually occur during the months of June, July, August and September, which correspond to the time of the year with the highest temperatures and driest conditions. By focusing our study only on these 4 months, we can have enough data to perform a valid statistical treatment, while avoiding a strong influence of PM10 from other sources in colder months (such as home heating or traffic). At the same time, we do not include in the analysis the deaths due to, for instance, cold and flu that could become confounding factors.

For the quantification, two steps were followed. First, an assessment of the incidence, patterns and variations of burned area on a large time frame and spatially integrated by NUTS III was done on the levels of air pollutants. PM10 and burned area were correlated through linear regression, while the mortality data and PM10 were correlated with Poisson regression. Data were processed and ordered by NUTS region and by month and year. Finally, the correlation between the pollutants emitted by forest fires, the wildfire-burned area and the different causes of mortality during the period 2001–2016 for the summer months were studied. The study is focused on PM since it is one of the main pollutants emitted by wildfires, which can increase PM concentrations by up to 50 % and more . Moreover, there is a clear relation with several effects on human health (including mortality), in particular with respiratory and circulatory diseases . There was not enough PM2.5 data collected from the Portuguese air quality management network to establish a correlation (only 20 stations measure PM2.5 on the mainland). For all these reasons this study focuses on PM10.

During the studied period (2001–2016), large fires were responsible for 46 % of the more than 2 million ha of forest burned in mainland Portugal. The areas most affected by number and size of wildfires are the north, centre and inland of the country. Wildfires do not follow a pattern in number of the occurrences or size during the years studied. This evidence was found despite the difficulties that the uneven scattering of the air quality monitoring stations analysing PM10 in Portugal posed. In fact, the areas where wildfires are usually more frequent (inland) are far from the urban centres (mainly along the coast) and thus not abundant in available air quality data due to the shortage (or even lack in some NUTS III regions) of monitoring stations. These regions also have an aged population, poorer economy and less health care resources, which can lead to an increase in the mortality rates in general. The socio-economic status of the population affected and the health care facilities and measures existing in the communities have to be taken into account , adding to the countless parameters that may affect these estimations which contribute to considerable gaps identified in this type of studies . Unfortunately, the scarce data available and the lack of accuracy in the existing data prevented us from estimating and/or including a correction regarding their influence.

Nevertheless, it was possible to find relationships between very relevant parameters. The significant positive correlation between PM10 and burned area found for 7 of the 13 NUTS III regions with available data is a good indication of where the influence of wildfires on the emission of PM10 is likely to be stronger. Although the location of the air quality monitoring stations may influence these correlations, especially when they are scarcer, for these NUTS regions it reveals the influence of wildfires on the local levels of PM10.

Large wildfires tend to be active for several days, releasing high amounts of pollutants to the atmosphere. In Portugal, such as in other Mediterranean countries, most of the wildfires are potentiated by strong winds which may spread the fire smoke over large distances . Thus, air quality monitoring stations located far from the ignition sites can still detect increases in the concentrations of, for instance, PM10. , while studying the impact of the uncontrolled wildfires of October 2017 in Portugal on mortality, found that the PM10 and PM2.5 emitted reached the United Kingdom, as well as other northern European countries. Likewise, in Finland, where ambient PM levels are relatively low compared to other countries in Europe, most of the strongest PM pollution episodes are typically related to emissions from wildfires in eastern European countries (Russia, Belarus, Ukraine, Estonia, Latvia and Lithuania) at a distance of hundreds to thousands of kilometres from southern Finland .

The negative impact particulate matter can bring to human health is well established and can be translated into several types of diseases . There is evidence that it can enter the human body, arrive in the bloodstream, and damage some organs or even cause death due to cardiovascular afflictions like stroke or heart attack, among others, representing a clear hazard to public health . Particulate matter also can damage the human respiratory system. The risk depends on the size of the particle, which if very small can even reach the alveolus .

In our study, the NUTS III regions where PM10 concentrations were found to be correlated with the burned area from large fires (Cávado, Ave, Tâmega e Sousa, Região de Aveiro, Viseu Dão-Lafões, Alto Tâmega and Beiras e Serra da Estrela) are indeed the ones where it would be expected to find the strongest influence of the wildfire-originated PM10 on population mortality. Although not for all, indeed associations between all-cause mortality and PM10 were found for three of these NUTS regions (Alto Tâmega, Viseu Dão-Lafões and Beiras e Serra da Estrela), with RRs varying from 1.003 to 1.006; between cardiovascular mortality and PM10 for one (Região de Aveiro) with an RR of 1.006, and between respiratory mortality and PM10 also for one (Viseu Dão-Lafões) with an RR of 1.020. Associations between cardiovascular mortality and PM10 were found for four NUTS regions (Alto Minho, A.M. Porto, Coimbra and Algarve) where there was no significant correlation of PM10 with burned area. All these are located on the coast, where the population density in Portugal is clearly predominant (particularly in A.M. Porto), as well as considerable industrial presence. In these regions, cardiovascular disease may have many other sources, some of them derived from a more sedentary and stressed lifestyle.

The mortality increase associated with PM10 is consistent with the estimates reported in other European studies, such as APHEA2 , APHENA , EpiAir and MED-PARTICLES , which also reported higher PM10 effects on all-cause, cardiovascular and respiratory mortalities.

However, some studies present uneven conclusions. reported the highest effects on cardiovascular mortality, but did not find any consistent effect with cardiovascular deaths in Australia, and registered the highest effects on respiratory mortality in Greece. This high variability may be related to several factors, notably (i) different PM composition or varying gaseous emissions (CO, VOCs, NOx or SO2) from wildfires, which may have different degrees of toxicity on cardiovascular and respiratory systems; or (ii) increasing temperature during wildfires, which is known to enhance the effects of PM on more susceptible individuals (e.g. cardiac patients) . Therefore, the effects we found on all-cause, cardiovascular and respiratory mortalities during the wildfire seasons may be due to different PM compositions or increasing temperature.

Region-specific associations between PM10 concentrations and mortality were also observed. These may have been influenced by the factors described above (different PM composition and increasing temperature) but also by the magnitude and duration of the exposure to PM from a given fire, the underlying health status of the population and the size of the population. The age of the exposed individuals can also be important. In some studies, larger effect estimates in groups of those 65 years and older have been reported. mentioned that the effect of respiratory mortality in Greece was higher in adults of ages 75 and above during large fires, whereas observed an increase in risk of cardiac arrest, especially in older adults in Australia, although not all resulted in death. In Brazil, reported that older adults had the strongest association between exposure to biomass burning and circulatory disease mortality. In Portugal, the regions traditionally impacted by wildfires coincide with a larger percentage of an aged population, which can help explain the obtained associations.

In our analysis, to study the relationship between the burned area and PM10 concentrations, averaged monthly data were used as the minimum temporal scale available for the burned area was 1 month. Other studies relating wildfire-originated PM and mortality are usually based on daily PM concentrations and daily death counts since they do not account for the burned area as a measure of the wildfire size. Therefore, the monthly approach obviously reduced the amount of data available and the possibility of finding more significant correlations, which may have diluted the effects of some wildfires on the population mortality. Moreover, some health effects may not have been detected because wildfires are episodic and local events. Nevertheless, the results provide an overall context, highlighting the strongest associations between wildfire-generated PM10 and all-cause, cardiovascular and respiratory mortalities. Being able to achieve them with this uneven distribution of available data is an indication that the approach can be very useful to at least uncover tendencies and, in regions with stronger monitoring capabilities and coverage, a way to find stronger and more accurate correlations. This will help legislators and other government bodies to propose ways to protect the population chronically exposed to wildfires or more susceptible to acute reactions to wildfire smoke.

Portugal is a country that suffers constantly from serious wildfire incidents, which are bound to pose a risk not only to chronically affected populations but also from acute impacts of the pollutants released in such events. In this work, analysing the summer months (June to September) on a lengthy time frame (2001–2016), it was possible to find relevant associations between PM10 (associated with large wildfires) and mortality in some NUTS III regions of mainland Portugal (mainly inland and in the north), as well as a significant correlation between burned area and PM10.

In particular, it was found that large fires (in this study considered above 1000 ha of burned area) have an impact on the health of the population in some areas due to the emission of particulate matter. The lack of data or possible confounding factors likely prevented a higher number of NUTS III regions with significant correlations. Moreover, in such severe events, the population exposed to a high concentration of pollutants in a short period of time should be considered as a risk modifier of the impacts of air pollution exposure .

These episodes occurred during the summer months (June–July–August–September) when high temperatures and long episodes of drought increase the probabilities of one of these extreme events. In a future ruled by climate changes, the high temperatures and long periods of drought that usually fuel big fires are expected to increase, thus paving the way for more extreme and intense events to occur even outside the typically affected regions. Thus, more population will be exposed more frequently to high pollutant levels, affecting their general health and increasing chronic diseases and mortality. Hence, restrictive policies and protocols to improve the effectiveness of preventive and mitigation actions must be enforced to face this environmental and societal issue.