Air Pollution Increases Influenza Hospitalizations

Gregor Singer; Joshua Graff Zivin; Matthew Neidell; Nicholas Sanders

To identify influenza hospitalizations, we count patients whose primary diagnosis is a strain of influenza. We use the Clinical Classifications Software (CCS) from the Agency for Healthcare Research and Quality (AHRQ) to classify relevant influenza ICD codes. These are the following ICD-9-CM codes: 4870, 4871, 4878, 488, 4880, 48801, 48802, 48809, 4881, 48811, 48812, 48819, 48881, 48882, 48889; and, for the period from October 2015 when the system was changed to ICD-10-CM, the following ICD-10-CM codes: J09X1, J09X2, J09X3, J09X9, J1000, J1001, J1008, J101, J102, J1081, J1082, J1083, J1089, J1100, J1108, J111, J112, J1181, J1182, J1183, J1189. We exclude patients whose primary diagnosis is not influenza, even if influenza is included among secondary diagnoses. Counting primary influenza diagnoses reflects a middle ground between two extreme alternatives for which we perform robustness checks. In one robustness check, we count patients who have any (primary or secondary) influenza diagnosis. In another robustness check we only count patients for whom influenza is their only diagnosis. We exclude patients whose zip code is from a different state than the hospital in which they are treated.