diff --git a/AstroAtmosphere/refractivityModels.py b/AstroAtmosphere/refractivityModels.py
index 1bcb1d9..1c58f2c 100644
--- a/AstroAtmosphere/refractivityModels.py
+++ b/AstroAtmosphere/refractivityModels.py
@@ -32,10 +32,10 @@ def BarrellAndSears(l, T=288.15, p=101325, H=0):
 	n = R/1e6 + 1
 	return n
 
-def Ciddor(l):
+def Ciddor(l, CO2=450):
 	'''
 		From Ciddor, 1996
-		Equations 1
+		Equations 1 and 2
 		At default conditions only. For more flexibility use
 		Observatory.n_tph()
 
@@ -47,6 +47,8 @@ def Ciddor(l):
 	'''
 	s = 1 / l
 	R = 5792105 / (238.0185 - s**2) + 167917 / (57.362 - s**2)
+	if CO2 != 450:
+		R = R * (1 + 0.534e-6 * (CO2 - 450))  
 	n = R/1e8 + 1
 	return n