Table 3. Hill's criteria [32] applied to thunderstorm-related asthma.

Hill's criteria	Application to the thunderstorm-related asthma
Temporal relationship	Thunderstorms always precede asthma attacks
Exposure always precedes the outcome
Strength	Increased risk of asthma attacks in relation to thunderstorms
The stronger the association, the more likely it is that the relationship of the factor to the health outcome is causal
Dose–response relationship
An increasing amount of exposure increases the probability of the health outcome Of note, the absence of a dose–response relationship does not rule out a causal relationship	Increased amount of pollen and mould spores at the beginning of the thunderstorm associated with increased probability of asthma attacks in pollen patients and other allergic patients
Consistency
The association is consistent when results in different studies and among different populations	Association between thunderstorm and asthma found in different studies and different populations
Plausibility
The association agrees with currently accepted understanding of pathological processes	Evidence of biological plausibility at the basis of thunderstorm-related asthma through pollen exposure (allergens and starch granules in the cytoplasm or other paucimicronic cytoplasmic-components carrying allergens) Evidence to be established in the case of mould spores
Consideration of alternate explanations	Alternate hypothesis involving chemical air pollution less explanatory than thunderstorm-related asthma
It is necessary to consider multiple hypotheses before making conclusions about the causal relationship between any two items under investigation
Experimental	Evidence indicating that prevention is possible by avoiding exposure to thunderstorm (at its beginning) in pollen patients
The condition can be altered (prevented or ameliorated) by an appropriate experimental regimen
Specificity	Poorly shown by experimental data (also sparse and heterogeneous)
This is established when a single putative cause produces a specific effect
Coherence	Existing theory and knowledge support the existence of thunderstorm-related asthma
The association should be compatible with existing theory and knowledge