Highly-sensitive optical fluorescent extruded plastic films are reported for the detection of gaseous and dissolved CO₂. The pH-sensitive fluorescent dye used is 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS, PTS⁻) coated on the surface of hydrophilic fumed silica and the base is tetrabutylammonium hydroxide (TBAH). The above components are used to create an HPTS pigment (i.e. HPTS/SiO₂/TBAH) with a high CO₂ sensitivity (%CO₂ (S = 1/2) = 0.16%) and fast 50% response (t_50↓) = 2 s and recovery (t_50↑) = 5 s times. Highly CO₂-sensitive plastic films are then fabricated, via the extrusion of the HPTS pigment powder in low-density polyethylene (LDPE). As with the HPTS-pigment, the luminescence intensity (at 515 nm) and absorbance (at 475 nm) of the HPTS plastic film decreases as the %CO₂ in the ambient gas phase increases. The HPTS plastic film exhibits a high CO₂ sensitivity, %CO₂ (S = 1/2), of 0.29%, but a response time <2 min and recovery time <40 min, which is slower than that of the HPTS pigment. The HPTS plastic film is very stable under ambient conditions, (with a shelf life >six month when stored in the dark but under otherwise ambient conditions). Moreover, the HPTS–LDPE film is stable in water, salt solution and even in acid (pH = 2), and in each of these media it can be used to detect dissolved CO₂.