Ambient pollution at hip fracture units and impact on mortality and post-operative delirium: A hormetic effect?

There is increasing awareness of the deleterious effects of ambient pollution. The World Health Organisation (WHO) has recently advocated new safe limits of annual exposure for the three pre-dominant pollutants: fine particulate matter (PM2.5), coarse particulate matter (PM10) and nitrogen dioxide; namely 5μg/m3, 15μg/m3 and 10gμ/m3 respectively. Both the USA and UK have recently implemented news standards which are lower than their current values, but still exceed those espoused by WHO. The WHO thresholds are challenging targets. It remains to be determined the proportion of secondary healthcare institutions located in zones with mean ambient pollutant levels in excess of the WHO limits and the impact this has on patients treated at these centres. This is particularly so for elderly patients who are theoretically most vulnerable to the adverse sequel of pollutant exposure. Using the UK National Hip Fracture Database and Defra Data (Department of Environmental, Food & Rural Affairs) we determined the annual mean PM2.5 PM10 and nitrogen dioxide exposure for all the units treating senescent hip fracture patients. We correlated these ambient pollutant levels with all-cause 30-day mortality and incidence of post-operative delirium for hip fracture patients. The vast majority (96%) of hip fracture units were located in zones where mean PM2.5 levels exceeded that required by the WHO guidance. A sizeable proportion also had annual mean exposures that surpassed the WHO PM10 (14.8%) and nitrogen dioxide (63%) recommended thresholds. There was no difference in 30-day mortality between hip fracture patients treated at units located in areas where pollutant titres were subliminal to the WHO guidance levels and those treated at centres where WHO thresholds were exceeded. By way of contrast patients admitted to institutions with mean ambient PM10 and nitrogen dioxide concentrations that surpassed the WHO limits had a lower risk of post-operative delirium compared to those at centres where the mean levels did not breach the WHO limit. For PM10 the relative risk was 0.89 CI:0.82-0.92 (p<0.0001) and that for nitrogen dioxide 0.92 CI: 0.89-0.94 (p<0.0001). The WHO target is ambitious as it relates to healthcare institutions. The majority are in areas that exceed WHO recommended limits. This does not appear to impact upon mortality for hip fracture patients. The decrease in incidence in post-operative delirium in areas of higher exposure raises, again in an epidemiological study, the possibility of the enigmatic phenomenon of hormesis, an adaptive response whereby low-dose exposure to a noxious agent or physiological stress enhances future physiological function.