Table 2.

Chronic obstructive pulmonary disease exacerbation rates and adjusted rate ratio in each season in different regions

	Summer	Fall	Winter	Spring
All regions
Event rate (events/person-month)	0.079	0.104	0.13	0.107
95% CI	0.072–0.87	0.096–0.112	0.12–0.139	0.098–0.116
Adjusted RR	1	1.31	1.64	1.34
95% CI		1.16–1.47	1.45–1.84	1.20–1.51
Northeast
Event rate (events/person-month)	0.077	0.11	0.15	0.11
95% CI	0.066–0.089	0.10–0.13	0.13–0.16	0.098–0.13
Adjusted RR	1	1.5	1.9	1.44
95% CI		1.22–1.79	1.54–2.29	1.19–1.75
Southeast
Event rate (events/person-month)	0.053	0.078	0.11	0.070
95% CI	0.057–0.073	0.059–0.10	0.087–0.14	0.051–0.093
Adjusted RR	1	1.47	2.1	1.32
95% CI		0.95–2.28	1.36–3.23	0.87–1.99
Upper Midwest
Event rate (events/person-month)	0.094	0.12	0.12	0.12
95% CI	0.078–0.11	0.098–0.14	0.098–0.13	0.1–0.14
Adjusted RR	1	1.23	1.39	1.31
95% CI		0.99–1.53	1.11–1.74	1.05–1.64
West
Event rate (events/person-month)	0.59	0.69	0.086	0.078
95% CI	0.043–0.079	0.052–0.09	0.066–0.11	0.059–0.1
Adjusted RR	1	1.18	1.47	1.34
95% CI		0.80–1.74	1.01–2.13	0.91–1.96
Southwest
Event rate (events/person-month)	0.084	0.09	0.12	0.11
95% CI	0.062–0.11	0.068–0.12	0.097–0.16	0.084–0.14
Adjusted RR	1	1.08	1.49	1.32
95% CI		0.72–1.61	1.06–2.11	0.88–1.98
Great Lakes
Event rate (events/person-month)	0.13	0.15	0.19	0.15
95% CI	0.095–0.17	0.12–0.19	0.13–0.25	0.11–0.2
Adjusted RR		1.17	1.42	1.10
95% CI		0.84–1.63	0.98–2.05	0.76–1.58

Definition of abbreviations: CI = confidence interval; RR = rate ratio.

Each season was defined as follows: winter (December to February), spring (March to May), summer (June to August), and fall (September to November). RR is adjusted for age, sex, forced expiratory volume in 1 second, oxygen use, smoking status, and body mass index