Table 2.

Selected male heat exposures and fecundability, restricted to first 3 cycles of follow-up (2013–2021)

Exposure	No. of Men	No. of Conceptions	No. of Cycles	Unadjusted FR (95% CI)	Adjusted FR (95% CI)a	Adjusted FR (95% CI)b	Adjusted FR (95% CI)c
Laptop usage on lap, hours/day
<1	2,178	941	4987	Reference	Reference	Reference	Reference
1	263	127	574	1.17 (0.99–1.37)	1.07 (0.90–1.26)	1.08 (0.91–1.27)	1.07 (0.91–1.26)
2	198	91	452	1.04 (0.86–1.27)	1.02 (0.84–1.25)	1.02 (0.84–1.24)	1.00 (0.82–1.22)
3–4	155	70	358	1.05 (0.85–1.31)	1.04 (0.84–1.29)	1.04 (0.84–1.29)	1.04 (0.84–1.29)
≥5	247	106	572	1.01 (0.84–1.21)	0.98 (0.81–1.17)	0.99 (0.82–1.19)	0.97 (0.81–1.17)
Sauna use, times/month
Never	2,849	1255	6487	Reference	Reference	Reference	Reference
1–2	141	55	336	0.87 (0.68–1.11)	0.84 (0.66–1.08)	0.86 (0.67–1.11)	0.87 (0.68–1.12)
≥3	51	25	120	1.08 (0.76–1.53)	1.03 (0.72–1.46)	1.04 (0.73–1.48)	1.04 (0.73–1.48)
Hot tub/bath use, times/month
Never	2,104	934	4782	Reference	Reference	Reference	Reference
1–2	714	317	1639	0.98 (0.87–1.10)	0.97 (0.87–1.09)	0.99 (0.88–1.11)	0.99 (0.88–1.11)
≥3	223	84	522	0.84 (0.68–1.03)	0.86 (0.70–1.05)	0.87 (0.70–1.07)	0.86 (0.70–1.06)
Use of seat heaters, hours/week
None	2,149	934	4897	Reference	Reference	Reference	Reference
0–2	535	249	1230	1.09 (0.90–1.30)	1.01 (0.89–1.14)	1.01 (0.89–1.14)	1.02 (0.90–1.15)
≥3	355	151	811	0.82 (0.63–1.08)	0.97 (0.83–1.13)	0.97 (0.83–1.14)	0.97 (0.83–1.14)
Type of underwear worn
Loose boxer/none	783	341	1812	Reference	Reference	Reference	Reference
Alternate loose/slim-fit briefs	333	135	775	0.93 (0.78–1.12)	0.94 (0.78–1.12)	0.94 (0.79–1.13)	0.94 (0.78–1.12)
Slim-fit briefs/boxer	1,921	857	4347	1.04 (0.93–1.17)	1.03 (0.92–1.16)	1.04 (0.93–1.16)	1.04 (0.92–1.16)
Time spent sitting, hours/day
<5	711	324	1612	Reference	Reference	Reference	Reference
5– 9	1,786	785	4099	0.96 (0.85–1.08)	0.94 (0.83–1.06)	0.94 (0.83–1.06)	0.93 (0.83–1.05)
≥10	544	226	1232	0.92 (0.79–1.07)	0.94 (0.80–1.10)	0.94 (0.80–1.10)	0.96 (0.82–1.13)
Fever episode in past 3 months
No	2,675	1178	6098	Reference	Reference	Reference	Reference
Yes	366	157	845	0.92 (0.77–1.10)	0.94 (0.79–1.12)	0.94 (0.79–1.12)	0.96 (0.81–1.14)
Truncated at 1 cycle of follow-up
No	2,675	552	2675	Reference	Reference	Reference	Reference
Yes	366	60	366	0.84 (0.64–1.10)	0.85 (0.65–1.11)	0.85 (0.65–1.11)	0.87 (0.67–1.14)
Cumulative heat metricd
0 heat factors	642	285	1481	Reference	Reference		Reference
1 heat factor	1393	610	3,170	1.01 (0.89–1.15)	0.99 (0.87–1.12)		0.99 (0.87–1.12)
2 heat factors	758	347	1733	1.02 (0.88–1.18)	1.03 (0.89–1.19)		1.04 (0.90–1.20)
3 heat factors	194	74	437	0.92 (0.73–1.17)	0.94 (0.74–1.19)		0.95 (0.75–1.20)
≥4 heat factors	54	19	122	0.81 (0.53–1.24)	0.77 (0.50–1.17)		0.79 (0.52–1.20)
Cumulative heat metrice
0 heat factors	646	285	1481	Reference	Reference		Reference
1 heat factor	1304	576	2987	1.02 (0.89–1.16)	0.99 (0.87–1.13)		0.99 (0.87–1.13)
2 heat factors	583	261	1305	1.04 (0.89–1.21)	1.04 (0.89–1.21)		1.04 (0.89–1.21)
3 heat factors	115	43	266	0.87 (0.64–1.17)	0.86 (0.64–1.16)		0.87 (0.64–1.17)
≥4 heat factors	27	13	59	1.08 (0.65–1.80)	1.00 (0.60–1.68)		1.01 (0.60–1.69)

^aAdjusted for age, race/ethnicity, education, total METs, alcohol intake, BMI, cigarette smoking, sleep duration, hours worked, unemployed, household income, and previously fathered a child.

^bMutually adjusted for other heat exposures when applicable, including sauna, bath, type of underwear, sitting, fever, laptop use on lap, and seat heater use.

^cFurther adjusted for female age, intercourse frequency, and female BMI.

^d“Heat factor” defined as ≥3 saunas/month, ≥3 hot baths/month, tight fitting underwear, sitting ≥10 hours/day, fever, laptop use ≥5 hours/day, or seat heater use ≥3 hours/week.

^e“Heat factor” defined as in footnote d, but without fever. Analyses are also restricted to males without fever in the 3 months before enrollment.