Table 1

Descriptive data at 1½ years of age for the baseline survey, the nested case–control study and persistence study

			Nested case-control study
	Baseline study		Control		Case		Persistence study
	N	Per cent	N	Per cent	N	Per cent	N	Per cent
Participants	63 266	100.0	3409	100.0	853	100.0	398	100.0
Gender
Boys	32 627	51.6	2132	62.5	533	62.5	272	68.3
Girls	30 554	48.3	1277	37.5	320	37.5	126	31.7
Wheezing at 1½ years
No	49 475	78.2	2659	78.0	367	43.0	–
Yes	13 791	21.8	750	22.0	486	57.0	–
Asthma at 1½ years
No	62 648	99.0	3409	100.0	853	100.0	–
Yes	618	1.0	–		–		398	100.0
Feeding method during the first 3 months of life
Bottle feeding	5716	9.0	326	9.6	72	8.4	44	11.1
Mixture	21 411	33.8	1143	33.5	292	34.2	128	32.2
Breast milk	35 875	56.7	1935	56.8	485	56.9	225	56.7
Pets at 1½ years
Pets (presently)	9305	14.7	465	13.6	154	18.1	64	16.1
Pets (previously)	2512	4.0	112	3.3	43	5.0	23	5.8
Never	50 787	80.3	2817	82.6	651	76.3	310	78.1
Familial smoking habits at 1½ years
Mother smokes	5591	8.8	258	7.6	96	11.3	51	12.9
Others smoke	13 637	21.6	701	20.6	193	22.6	82	20.8
No smokes	43 469	68.7	2430	71.3	554	64.9	262	66.3
History of allergic diseases*
Yes	10 975	17.3	573	16.8	268	31.4	135	34.5
No	48 095	76.0	2640	77.4	547	64.1	256	65.5
Parental history of allergic diseases
Yes	37 583	59.4	2074	60.8	653	76.6	319	81.8
No	23 096	36.5	1215	35.6	175	20.5	71	18.2
Outdoor concentration of NOx at 1½ years (ppb)†
50.9–136.8 (level 5)	3186	5.0	205	6.0	41	4.8	29	7.3
42.1–50.8 (level 4)	12 658	20.0	647	19.0	181	21.2	108	27.1
37.2–42.0 (level 3)	15 856	25.1	874	25.6	214	25.1	101	25.4
32.6–37.1 (level 2)	15 801	25.0	820	24.1	197	23.1	89	22.4
13.9–32.5 (level 1)	15 765	25.0	863	25.3	220	25.8	71	17.8
Outdoor concentration of EC at 1½ years (μg/m³)‡
3.6–7.5 (level 5)	3958	6.3	222	6.5	48	5.6	26	6.5
3.1–3.5 (level 4)	16 389	25.9	839	24.6	232	27.2	90	22.6
2.8–3.0 (level 3)	15 430	24.4	839	24.6	204	23.9	82	20.6
2.5–2.7 (level 2)	15 787	25.0	893	26.2	217	25.4	107	26.9
1.3–2.4 (level 1)	11 702	18.5	616	18.1	152	17.8	93	23.4
Distance from traffic, main road at 1½ years
<50 m	2394	3.8	138	4.0	29	3.4	17	4.3
50 m to <100 m	2700	4.3	144	4.2	37	4.3	21	5.3
100 m or more	58 172	91.9	3127	91.7	787	92.3	360	90.5
Birth season
December–February	15 488	24.5	976	28.6	244	28.6	88	22.1
March–May	18 338	29.0	920	27.0	230	27.0	129	32.4
June–August	18 616	29.4	929	27.3	233	27.3	116	29.1
September–November	10 824	17.1	584	17.1	146	17.1	65	16.3
Years of residence
Below 1 year	9178	14.5	484	14.2	111	13.0	49	12.3
Over 1 year	7161	11.3	383	11.2	77	9.0	30	7.5
The same since birth	46 672	73.8	2539	74.5	663	77.7	317	79.6
Structure of house
Wooden house	16 381	25.9	987	29.0	250	29.3	125	31.4
Steel frame building	46 113	72.9	2392	70.2	595	69.8	273	68.6
History of pneumonia
Yes	2833	4.5	137	4.0	100	11.7	87	21.9
No	60 433	95.5	3272	96.0	753	88.3	311	78.1
History of empyema
Yes	599	0.9	32	0.9	16	1.9	7	1.8
No	62 667	99.1	3377	99.1	837	98.1	391	98.2
Heating system
Without ventilation	19 533	30.9	1081	31.7	292	34.2	133	33.4
With ventilation	2512	4.0	119	3.5	34	4.0	25	6.3
No heating system	40 588	64.2	2184	64.1	522	61.2	239	60.1
Districts
Chiba	3213	5.1	216	6.3	54	6.3	23	5.8
Itabashi	4303	6.8	248	7.3	62	7.3	36	9.0
Suginami	3476	5.5	260	7.6	65	7.6	34	8.5
Setagaya	5402	8.5	248	7.3	62	7.3	26	6.5
Kawasaki	9930	15.7	688	20.2	172	20.2	62	15.6
Yokohama	13 250	20.9	772	22.6	193	22.6	97	24.4
Nagoya	9493	15.0	388	11.4	97	11.4	52	13.1
Osaka	9260	14.6	397	11.6	100	11.7	48	12.1
Kobe	4939	7.8	192	5.6	48	5.6	20	5.0
Chiba	3213	5.1	216	6.3	54	6.3	23	5.8

*History of allergic diseases before the age of 1½ years.
†The NOx concentrations were classified as levels 5–1 (top 5, 6–25%, 26–50%, 51–75% and 76–100%, respectively).
‡The EC concentrations were classified as levels 5–1 (top 6, 7–32%, 33–56%, 57–81% and 82–100%, respectively).
EC, elemental carbon; NOx, nitrogen oxides.