Higher temperature and air pollution levels linked with PCP hospitalisations among people with HIV

Temperature and air pollution levels are associated with hospitalisations caused by Pneumocystis pneumonia (PCP) in people with HIV, investigators from San Francisco report in the online edition of Clinical Infectious Diseases. Hospitalisations due to PCP were highest in the summer months and there was a significant association with an increase in temperature and higher levels of sulphur dioxide (SO₂).

"The identification of both climatological and air pollution constituents associated with the development of PCP is a novel and important observation", write the authors.

PCP is an AIDS-defining opportunistic infection. It was a major cause of death in people with HIV in the era before effective antiretroviral therapy was introduced, and the infection remains the second most common AIDS-defining illness in the US.

Environmental factors such as temperature and air pollution have been associated with an increased risk of a number of lung disorders. However, the role of climate and air pollution in the development of PCP are poorly understood.

Investigators from San Francisco General Hospital therefore designed a study involving 457 people with HIV admitted to their facility between 1997 and 2008 with confirmed PCP.

Information on climate and air pollution levels one week, one month and two months before each hospitalisation were obtained by the investigators.

Most of the patients (89%) were men, 48% were white and the median age at the time of admission to hospital with PCP was 40 years. The patients had advanced HIV disease with a median CD4 cell count of just 31 cells/mm³. Only 61 (13%) had received PCP prophylaxis in the three months before hospitalisation.

Rates of admission differed significantly (p < 0.05) according to season of the year, and were highest in summer (129 hospitalisations), followed by spring (125 admissions). There were only 91 admissions in the winter months.

There was a significant association between increases in temperature and PCP hospitalisations. Each 5^oF increase in temperature increased the risk of admissions by 41% (OR = 1.41; 95% CI, 1.14-1.75).

Air pollutant levels were also associated with hospitalisations. A one-unit increase of SO₂ parts per billion increased the risk of admissions by 80% (OR = 1.80; 95% CI, 1.15-2.83).

An analysis of environmental conditions two weeks before hospitalisation showed that increases in temperature (p < 0.01) and higher SO₂ (p < 0.001) both had a significant association with PCP admissions.

Higher temperature (p = 0.03) and SO₂ (p < 0.01) one month before admission were also significant risk factors. There was also a significant association with temperature two months before hospitalisation (p < 0.01).

The authors suggest “SO₂ exposure in the patients described in the present study contributed to an impairment of pulmonary defence mechanisms, and aggravated pre-existing and evolving PCP symptoms resulting in these patients seeking medical care.”

An unexpected result was that higher carbon monoxide levels were found to be protective and reduced the effects of SO₂. “It is difficult to find a good explanation for this association,” comment the investigators.

They conclude, “Further multicenter studies are needed to identify if these factors are also predictors of PCP admissions in other geographical locations and in other immunocompromised groups. Animal studies are also needed to better understand the biological mechanisms behind the impact of climatic air pollution on PCP occurrence.”

The rigour of the study was praised in an accompanying editorial. “This is an important area of study and hopefully the authors will continue to pursue the role of the environment on the colonization, infection and pneumonia from Pneumocystis.”