Table 3.

Cluster analysis of literature from 1999–2018.

Cluster ID	Size	From	To	Activeness	Theme
0	64	2007	2017	Active	Household air pollution
1	61	2001	2009	Inactive	Nitrogen dioxide
2	60	1995	2004	Inactive	Preterm delivery
3	29	1994	1999	Inactive	Outdoor air pollution
4	29	2001	2010	Inactive	Pregnancy outcome
5	25	2005	2015	Active	Smoke-free legislation
6	24	1994	2002	Inactive	High endotoxin level
7	22	1995	2001	Inactive	Parental smoking
8	20	2008	2015	Active	Autism spectrum disorder
9	15	2012	2017	Active	Early childhood ear infection
10	10	2002	2006	Inactive	Indoor environmental influence