Our results indicate that the daily incidence of dog bites is influenced by multiple environmental variables, including ozone, temperature, precipitation, and UV levels, but not PM 2.5 . Sensitivity analyses indicated that these relationships were stable and not greatly influenced by models or co-variance between variables. This is in line with prior studies on the impact of ozone on human aggression13,15,17,18, and studies on human aggression and temperature36. The effect of UV irradiation increasing aggression is in line with recent studies indicating increased aggression and increased sex-steroid levels after UVB exposure in mice and men37.
Ozone has a strong smell, is highly reactive and triggers oxidative stress in the airways and impairs pulmonary function. Due to its reactivity, ozone is not thought to penetrate beyond the membranes lining the respiratory tract and lungs, so behavioral effects may occur via generation of free radicals from lipid peroxidation. In humans, ozone exposure triggers the release of multiple messenger pathways, including serum amyloid A38, interleukin-639,40 and interleukin-841 and activation of the hypothalamic pituitary adrenal (HPA) axis42. Behavior may thus be influenced by a general stress response to pollutants triggered by lung inflammatory messengers. More direct effects on brain function are also possible: In rats, acute ozone exposure rapidly increases dopamine43, noradrenaline, dihydroxyphenylacetic acid, and 5-hydroxyindolacetic acid in the striatum and midbrain44. Ozone exposure further stimulates catecholamine biosynthesis in the hindbrain noradrenergic A2 group, catecholamine turnover is increased in the cortex, but decreased in the striatum45. In human experimental ozone exposure studies, 4 h of 200 ppb ozone exposure led to a 79% increase in 8-isoprostane (8-ISO), a measure of lipid oxidation, 18 h after exposure46. Notably, 8-ISO levels are elevated in intermittent explosive disorder, and further correlated to measures of actual aggressive behaviors47. As the neural circuitry for aggressive behaviors is conserved across mammals and given the impact of ozone on basal ganglia dopaminergic function, we speculate that ozone may influence aggressive behavior via impacts on dopamine turnover in the striatum. While combustion derived PM 2.5 has been detected in the brains of both dogs48 and humans49, we did not observe and effect of PM 2.5 on dog bite incidence. Compared to humans, dogs have a much larger surface area of olfactory epithelium, more olfactory receptors, and a larger olfactory bulb50,51. As such, anatomical differences between humans and dogs may account for the lack of effect in this study.
We utilized animal control and hospital records to evaluate the impact of temperature and air pollutants on dog bite incidence. However, survey data indicates that the true burden of dog bites is much higher than reported in hospital data52 and only a small percentage of dog bites require extensive medical treatment or hospitalization53. Our results are therefore likely indicative of more severe dog bite incidents. According to prior studies, most dog bites arise from a dog known to the victim, and most bites are related to interacting or attempting to interact with the dog21,25. While it is likely that human–dog interactions increase on days with higher temperature and higher UV irradiation (i.e., sunny days), our analysis indicates that ozone levels further contribute to the risk of dog bites, an effect present in both winter and summer months independently. Moreover, our analysis indicates a slight decreased risk on weekends and holidays, suggesting that ample time for dog–human interactions does not increase risk.
A limitation of our analysis is that public records of dog bites do not provide more detailed information about dog breed, sex, castration/spaying status, nor for bite severity, victim age, gender, familiarity with dog and the interactions leading up to the dog bite, all factors that impact the risk and consequence of dog bites21,23,24,25.
We included data spanning 2009 to 2019. Earlier datapoints were not publicly available from our sources. We did not include data from the COVID-19 era. During COVID-19 lockdowns, air pollution decreased, but pediatric emergency department visits for dog bites increased54,55. This suggest that other factors, such as forced proximity, may be a larger determinant in dog-on-human aggression. According to the American Veterinary Medicine Association, dogs bite primarily as a reaction to something, such as stressful situations, a scare, startle, or threat, or to protect food, toys or their puppies56. Dogs might bite defensively or to be left alone22,24,25. In our analysis, it is unclear if dog behavior is directly altered by ozone and heat, or, if the observed increase in dog bites is a consequence of altered behavior imposed by the human victim and/or the dogs master, which in many cases are the same individual21.
The effects of increasing temperature and air pollutants on human aggression, as indexed by police records, are well established5,6,7,12,13,14,15,16,17. Yet police records of criminal activity, while extensive and well documented, may have systematic biases: less than 45% of violent crimes are reported to law enforcement57. Criminal reporting may further be impacted by the behavior of victims and bystanders, as well as by the priorities and resources of law enforcement. The present findings, expand the association between temperature, air pollutants and aggression across species to also include dogs. It is notable that in rodents, exposure to ozone, heat stress, and their combination induces cognitive decline and neuroinflammation58. The link between ozone and aggression awaits verification such as by randomized double blinded exposure experiments in animals or possibly humans. While cardiovascular and pulmonary health effects of pollution are well established, the present results emphases the impacts on behavior and mental health. Through such mechanism, air pollutants and extreme heat could contribute to higher societal and individual burdens then currently appreciated.