The Impact of Local Air Quality on the Number of Hospital Admissions with Acute Pain in Sickle Cell Disease within an Urban Environment.

The clinical severity of sickle cell disease (SCD) is dependent on genetic and environmental variables. The impact of environmental factors on disease is a major public health issue and air pollution has been consistently correlated with poor health outcomes. Environmental factors in SCD have been poorly studied. We have retrospectively studied the numbers of daily admissions with vaso-occlusive sickle cell pain to King’s College Hospital, London, in relation to local daily air quality measurements. We analysed 1047 patient episodes over 1400 days (1^st January 1998 to 31^st October 2001). Statistical time series analysis was performed using cross-correlation function (CCF), where the observations of one series are correlated with the observations of another series at various lags and leads, values >0.05 being significant. This showed a significant association between increased numbers of admissions and low levels of nitric oxide (NO) (CCF=0.063), low levels of carbon monoxide (CO) (CCF=0.064) and high levels of ozone (O3) (CCF=0.067). There was no association with sulphur dioxide (SO2), nitric dioxide (NO2) or dust. The significant results were further examined using quartile analysis. This confirmed that increased numbers of hospital admissions were associated with high levels of O3 (oneway ANOVA p=0.039) and low levels of CO (oneway ANOVA p=0.042). Low NO levels were also associated with increased admissions, not however reaching statistical significance on quartile analysis (oneway ANOVA p=0.158). O3 levels show marked seasonal variation, with high levels occurring in summer months in the UK. We have previously shown a trend towards increased admissions in the summer months with acute sickle related pain, whereas other groups, which are primarily based in rural tropical climates, found increased admissions in cold, rainy months. The adverse effect of high O3 levels may explain this difference. There is no direct evidence to explain the mechanism by which high O3 levels leads to vaso-occlusion in SCD but high levels of O3 have been linked to reduced respiratory function, which may in turn precipitate vaso-occlusion. Independent studies have shown high CO levels are linked to increased respiratory and cardiovascular admissions. Paradoxically we found that high CO levels were linked to decreased admission numbers and may be protective against acute pain in SCD. CO may confer benefit by forming carboxyhaemoglobin that cannot polymerise resulting in decreased sickling. Previous studies have shown prolonged red cell survival in vivo following administration of CO to patients with SCD. Our study also suggests higher levels of atmospheric NO are linked to fewer admissions. NO is known to be central in the pathophysiology of vaso-occlusion and sickle cell patients are thought to have functional deficiency of NO. Many groups have reported inhaled NO as beneficial in the treatment of sickle pain. Our study suggests air quality has a significant effect on acute pain in SCD and that patients should be counselled accordingly. Based on these findings it would be appropriate to warn patients that high O3 levels might precipitate complications of SCD. The potential beneficial effect of CO and NO is intriguing and requires further investigation.