Table 3.

Prevalence and association of dermatomyositis (DM) with sunburn and sun exposure in the year prior to diagnosis, by maximum ambient ultraviolet radiation (UV) radiation levels based on residential location at diagnosis*

	At or above the median of maximum UV dose				Below the median of maximum UV dose
	DM	PM/IBM	DM vs. PM/IBM		DM	PM/IBM	DM vs. PM/IBM
Effect	n (%)	n (%)	OR (95% CI)	P	n (%)	n (%)	OR (95% CI)†	P	Interaction P
Sunburn
≥ 2 sunburns	66 (21)	50 (15)	1.52 (0.97–2.38)	0.066	66 (22)	40 (11)	2.05 (1.29–3.27)	0.002	0.468
1 sunburn	63 (20)	47 (14)	1.62 (1.04–2.54)	0.034	61 (20)	58 (17)	1.25 (0.82–1.92)	0.303	0.350
No sunburn	184 (59)	246 (72)	1.00 (REF)		175 (58)	252 (72)	1.00 (REF)
Combined sun exposure
High/Mod-High	35 (13)	40 (14)	1.89 (1.03, 3.48)	0.040	35 (14)	39 (13)	1.34 (0.75, 2.41)	0.329	0.673
Moderate	151 (58)	140 (50)	1.64 (1.09, 2.47)	0.017	136 (55)	146 (51)	1.13 (0.76, 1.67)	0.546	0.259
Low/None	75 (29)	98 (35)	1.00 (REF)		78 (31)	104 (36)	1.00 (REF)

Abbreviations: DM, dermatomyositis; PM, polymyositis; IBM, inclusion body myositis; OR, odds ratio; CI, confidence interval; REF, referent group.