Study Type of
orofacial cleft		 CO NOx O3 PM10 SO2
Ritz et al. [12] CL/P I ▬ I ▬ ▬
CP I ▬ I ▬ ▬
Chung et al. [17] CL/P ▬ ↓ ▬ ▬ ▬
CP ▬ ↓ ▬ ▬ ▬
Dolk et al. [18] CL/P ▬ ↓ ▬ ↓ ↓
CP ▬ ↓ ▬ ↓ ↑
Gilboa et al. [19] CL/P ↑ ↓ ↑ ↑ ↓
CP ↑ ↑ ↑ ↑ ↑
Hansen et al. [20] CL/P ↓ ↑ ↓ ↑ ↑
CP ↓ ↓ ↓ ↓ ↓
Hwang et al. [21] CL/P ↔ ↓ ↑ I ↓
CP ↔ ↓ ↑ ↔ ↓
Marshall et al. [22] CL/P ↑ I ↑ I ↑
CP ↓ I ↑ I I
Rankin et al. [23] CL/P ▬ ▬ ▬ ▬ I
CP ▬ ▬ ▬ ▬ ▬

I = inconsistent results; ▬ = outcome not measured; ↑ = increased; ↓ = decreased; ↔ = no change.