Table 2.

Associations between nitrate and total trihalomethanes (TTHM) in public water supplies and incident colon and rectal cancers, Iowa Women’s Health Study participants >10 years at their drinking water source (N=15,532^a)

			Colon		Rectum
	N	Cases	HR^b, 95%CI	Cases	HR^b, 95%CI
Average NO₃-N (mg/L)
≤0.36	3,226	118	1.00 (Ref.)	39	1.00 (Ref.)
0.37–0.80	3,055	126	1.13 (0.88, 1.45)	18	0.48 (0.28, 0.84)
0.81–1.35	3,020	146	1.32 (1.03, 1.69)	35	0.86 (0.53, 1.38)
1.36–3.51	3,251	116	0.98 (0.76, 1.27)	39	0.94 (0.60, 1.48)
>3.51	2,980	106	0.97 (0.75, 1.26)	24	0.64 (0.38, 1.07)
p _trend			0.18		0.69
Continuous^c			0.97 (0.90, 1.05)		0.93 (0.80, 1.08)
Years >5 mg/L NO₃-N
0		447	1.00 (Ref.)	111	1.00 (Ref.)
1–4	10,891	96	0.91 (0.73, 1.13)	25	0.97 (0.63, 1.50)
≥4	2,613	69	0.82 (0.63, 1.06)	19	0.86 (0.53, 1.41)
p _trend	2,028		0.10		0.60
Private well ^d		246	1.04 (0.89, 1.21)	64	1.08 (0.80, 1.45)
Average TTHM (μg/L)
≤0.68	3,143	189	1.00 (Ref.)	37	1.00 (Ref.)
0.69–2.17	3,191	149	0.87 (0.67, 1.12)	33	1.28 (0.73, 2.20)
2.18–7.82	2,973	135	1.10 (0.85, 1.41)	35	1.63 (0.94, 2.84)
7.83–17.7	3,130	118	0.87 (0.67, 1.13)	41	1.83 (1.06, 3.16)
>17.7	3,095	147	1.13 (0.89, 1.44)	40	1.71 (1.00, 2.92)
p _trend			0.11		0.22
Continuous^c			1.01 (0.98, 1.05)		1.06 (0.99, 1.14)
Years >40 μg/L TTHM
0	11,768	440	1.00 (Ref.)	110	1.00 (Ref.)
1– 35	1,946	91	1.26 (1.01, 1.58)	26	1.43 (0.93, 2.20)
≥36	1,818	81	1.23 (0.97, 1.56)	19	1.13 (0.69, 1.84)
p _trend			0.07		0.36

After excluding 378 participants with missing covariate data.

All models adjusted for age, physical activity, smoking status, and mutually adjusted for NO₃-N or TTHM.

HR per 1 unit increase in the natural log-transformed value.

Reference group is women in Q1 of average NO₃-N.