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

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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.


Pneumocystis carinii pneumonia (PCP)

Pneumocystis carinii pneumonia is a form of pneumonia that is an AIDS defining illness.


Any lung infection that causes inflammation. The infecting organism may be bacteria (such as Streptococcus pneumoniae), a virus (such as influenza), a fungus (such as Pneumocystis pneumonia or PCP) or something else. The disease is sometimes characterised by where the infection was acquired: in the community, in hospital or in a nursing home.

AIDS defining condition

Any HIV-related illness included in the list of diagnostic criteria for AIDS, which in the presence of HIV infection result in an AIDS diagnosis. They include opportunistic infections and cancers that are life-threatening in a person with HIV.


Affecting the lungs.



Having a weakened immune system, therefore, a reduced ability to fight infections and other diseases.

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 5oF 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.”


Djawe K et al. Environmental risk factors for Pneumocystis pneumonia hospitalizations in HIV patients. Clin Infect Dis, online edition, 2012.

Hughes WT Environmental risk factors for Pneumocystis pneumonia hospitalization in HIV/AIDS patients. Clin Infect Dis, online edition, 2012.