Toxic gases and particulate matter (such as CO2, NO2 etc.) account for millions of deaths every year due to respiratory-related issues. Wearable gas sensors offer a good platform for continuously monitoring the quality of air can give information about the degree of air pollution at any location and time. This will allow the wearer to monitor their exposure and to participate in crowd sensing air pollution in which the data can be shared and used for mapping polluted areas and enable safety measures for preventing and mitigating the effect of these gases on humans and their environment. In this regard, e-textiles provide an attractive platform for a wearable gas sensor. This work builds upon previous research conducted at the University of Southampton that developed a flexible circuit fabrication and packaging technology that enables the circuits to be woven into a textile. The packaging process previously demonstrated used a conformal polymer (Kapton) film bonded to the substrate but for an e-textile gas sensor, it is important that this packaging is gas permeable whilst remaining flexible and waterproof. There have not been any reported approaches for the encapsulation of flexible gas sensors with thermoplastic films previously. This work focuses on investigating the various techniques and flexibility of the materials that are waterproof and gas permeable for addressing the above issue. A flexible strip with a MOX gas sensor was built to detect carbon monoxide. A novel way of packaging flexible electronics using vacuum forming was achieved using breathable and waterproof thermoplastic polyurethane film. The sensor was also encapsulated with two different thermoplastic materials – microporous polytetrafluoroethylene Zitex G – 104 and PTFE glass fibre fabric using flexible jigs. Zitex G – 104 and PTFE fibre glass fabric encapsulated flexible sensor have negligible effect on the sensor response to Carbon monoxide whereas TPU acts as a gas barrier film. It was found that the Zitex G – 104 packaged sensor has the highest mechanical robustness to bending and washing cycles.