The effect of heat waves on mortality is well documented.7 It has been observed a rapid rise in the number of hot days and severe meteorological events,89 such as the 2003 and 2012 heat waves when temperatures reached or went above 35℃ degrees, resulting in excess deaths across Europe.89 In Europe, over 70,000 excess deaths were observed in 12 European countries in the heatwave summer of 2003. Such high summer temperatures are expected to be the norm by the middle of the century in Europe.10 There is evidence of an increased number of deaths and acute morbidity, especially among respiratory patients due to heat waves. For every 1℃ rise, the risk of premature death among respiratory patients is up to 6 times higher than in the rest of the population.6 The increase in respiratory mortality (relative risk) is larger than total or cardiovascular mortality.11 Although the association between heat and the number of hospital admissions is less studied and less evident, admissions are also more apparent for respiratory than for cardiovascular disease.8 Moreover, climate scenarios for the next century predict that the warming will be associated with more frequent and more intense heat waves in wide areas of our planet with increased risk of wildfires and desertification. Heat and drought conditions contribute to wildfire risks. Smoke emissions can travel hundreds of kilometers downwind of fire areas, exposing people to a complex mixture of fine particles, ozone precursors, and other health-harming compounds1213 One recent worldwide estimate is that 339,000 deaths annually may be attributed to landscape fire smoke.13 Respiratory and cardiovascular hospital admissions and emergency department visits increase in response to wildfire smoke exposure, strongly associated with PM levels.1314 Drought conditions create multiple health challenges: in dry conditions, more pollen, dust, particulates, and when present, wildfire smoke which can irritate respiratory epithelium, exacerbate chronic respiratory illnesses, and asthma, and increase risks for acute respiratory infection (Table 1).15 In urban areas, the effects are higher because climate change influences outdoor air pollution because the generation and dispersion of air pollution is in strict correlation with local patterns of temperature, wind, and precipitation.16

Air pollution is an alteration in the levels of quality and purity of the air due to natural or anthropogenic emissions of chemical and biological substances. It is the environmental factor with the greatest impact on respiratory health in Europe.12 Pollutants affecting air quality include inhalable particles. To put it simply, inhalable particles can be defined as those whose aerodynamic diameter is less than 10 µm (PM10), while fine inhalable particles are defined as less than 2.5 µm (PM2.5) and coarse inhalable particles, between 2.5 and 10 µm.17 The size of the particle, and surface and chemical composition of the particulate material determine the risk that exposure to this agent represents for human health. Fine particles, owing to their smaller size, penetrate deeply into the respiratory system and may affect the alveoli. Air pollutants show these effects by causing direct cellular injury or by inducing intracellular signaling pathways and transcription factors that are known to be sensitive to the oxidative stress.181920212223

Degradation of air quality caused by 1 or more pollutants, which concentration levels and presence time vary, has been related to asthma. Outdoor air pollution exacerbates asthma in those who already have the condition.24 Outdoor levels of air pollutants have been associated with asthma incidence but not neatly with asthma prevalence at the population level.

Particle pollution, vehicle exhaust, and ground-level ozone are the most important types of hazardous pollutants. Ozone is a strong oxidant that has been associated with persistent structural airway and lung tissue damage, as well as contributing to more severe symptoms of asthma and an increase in respiratory hospital admissions and deaths in Europe and the USA.16 It is estimated that there will be an annual ozone level of >1,500 ppm associated deaths by the year 2020 in the UK alone.16 Pollution models for climate change scenarios predict an increase in ozone concentrations over large areas, while the effect on particle concentrations is less clear.24 The short-term effects of ozone on daily mortality and respiratory disease are extensively studied, while there is only limited documentation of long-term effects on mortality.2425

Positive associations have been observed between urban air pollution and respiratory symptoms in children, and the literature contains many reports of relations between motor vehicle exhausts and acute or chronic respiratory symptoms in children living near traffic.242526

Air pollution can negatively influence lung development in children and adolescents.252728 Most studies have suggested adverse effects of air pollution on children's lung function and respiratory symptoms. Deficits in lung function were associated with a correlated set of pollutants, including nitrogen dioxide, acid vapor, fine particulate matter (PM2.5), and elemental carbon. Deficits in lung function during young adulthood may increase the risk of respiratory conditions, such as episodic wheezing that occurs during a viral infection. However, the greatest effect of pollution-related deficits may occur later in life, since reduced lung function is a strong risk factor for complications and death during adulthood.29 Prediction about the effects of climate change on health-related air pollution is hampered by several limits: future emissions depend on numerous factors, such as population growth, economic development, energy use, and production; current knowledge about weather effects on air pollution is still unsatisfactory; there is still a need of better emission inventories and observational datasets; long-term effects and actual enforcement of international agreements to reduce air pollution and greenhouse gases emissions (e.g. Kyoto protocol) are unpredictable.

Regarding indoor pollution, indoor levels of air pollutants other than environmental tobacco smoke have also been related to asthma prevalence or symptoms by sparse studies.303132

Consistent results support short-term (aggravation) and, although more rarely, long-term (prevalence augmentation) effects on asthma of poor air in indoor settings.303132 Environmental tobacco smoke is one of the most important risks for respiratory symptoms and diseases worldwide. The evidence is also reliable for indoor nitrogen dioxide and particulate matter, which have been associated with asthma. Whereas formaldehyde and volatile organic compounds seem to be the main pollutants in indoor settings, relevant papers on asthma are still scarce and limited to asthma and bronchitis. Molds have been associated with an increased risk of asthma and COPD.

The effects of climate change on respiratory allergy are still unclear, and studies addressing this topic are lacking. Global warming is expected to affect the start, duration, and intensity of the pollen season on the one hand, and the rate of asthma exacerbations due to respiratory infections and/or cold air inhalation on the other.33 Data provided by 30 years of observations within the International Phenological Gardens Network showed that spring events advanced by 6 days, the highest rate of phenological changes being observed in Western Europe and Baltic regions. Conversely, phenological trends appear to be different in the eastern border of Europe, sometimes showing a 1-2 weeks later start of the phases. Due to the earlier onset, the pollen seasons are more often interrupted by adverse weather conditions in late winter/early spring.333435 Duration of the pollen season is also extended, especially in summer and in late flowering species. The climate change projected during next century will influence plant and fungal reproductive systems and alter the timing, production, and distribution of aeroallergens. Increased exposure to allergens as a result of global warming, combined with exposure to pollutants that act synergistically to intensify the allergic response, could point to increased respiratory problems in the future. In fact, climate change is likely to influence vegetation, with consequent changes in growth and reproductive cycles and in the production of allergenic pollen (seasonal period and intensity). In addition, weed species are expected to proliferate. These changes can vary from one region to another, since some areas receive greater amounts of UV radiation and/or rainfall than others (Table 2).

An increasing body of evidence shows the occurrence of severe asthma epidemics during thunderstorms in the pollen season, several epidemics of asthma have been reported following thunderstorms in various geographical zones, prevalently in Europe and Australia.36 Asthma epidemics related to thunderstorms are limited to seasons when there are high atmospheric concentrations of airborne allergenic pollens (subjects with pollen allergy who stay indoors with the window closed during thunderstorms are not involved); further there are no observations on the involvement of asthma in nonallergic subjects. Much remains to be discovered about the relationship between asthma attacks and thunderstorms, but there is reasonable evidence in favor of a causal link between them in patients suffering from pollen allergy. In particular, during the first phase of a thunderstorm, that is, the first 20-30 minutes, there is evidence of high respirable allergen loadings in the air. This is due to dry updrafts that, during thunderstorms, entrain whole pollens into the high humidity at the cloud base where pollens may rupture and cold downdrafts carry pollen fragments (pollen grains are too large to penetrate the deeper airways) to the ground level where outflows distribute them. Due to strong electric fields that develop during thunderstorms, positive ions are released from the ground and could attach to particles and/or electric charge may enhance pollen rupture, thus enhancing bronchial hyperresponsiveness.36373839

In the light of the above, subjects affected by pollen allergy-not only asthmatic patients but also subjects affected by seasonal rhinitis without asthma symptoms-should be alert to the danger of being outdoors during thunderstorms in the pollen season, as such events may be an important cause of severe exacerbations (Table 3).