Carbon dioxide produced by fossil fuel burning, cement manufacture, and deforestation contributes greatly to the greenhouse effect, which is one of the most important consequences of industrialization. The Internet of Things (IoT), wireless communication networks and sensor devices, enables real-time tasks and data interchange in a central server with high-performance and cost-effective components. We design and develop an IoT and cloud-based system to monitor CO 2 levels in the environment in two separate places in Dhaka city buildings: Rooftop Garden Area (RGA) and Normal Regular Rooftop (NRR) without a garden. The difference in carbon dioxide levels between the two sites varies throughout a number of days. The data is collected in four dayparts: Early Morning (EM), Morning (M), Noon (N), and Afternoon (AN) at three-hour intervals. The MQ-135 gas sensor monitors long-term and large-scale carbon dioxide levels, and the collected data is sent to the Firebase cloud storage using the NodeMCU ESP 8266 12E Wi-Fi module. To ensure that the internet connection is always available and that dummy values are avoided, the system in the network was continuously monitored. Data is collected at 5-minute intervals from the beginning of the morning (6:00 AM) until the end of the afternoon (6:00 PM) to obtain the most accurate value. We took 432 data row points and visualized them using a variety of comparison themes. According to the statistics, the maximum amount of CO 2 in a regular rooftop was 568 PPM, whereas the maximum level of CO 2 in a rooftop garden area was 443 PPM. That paper developed a very dependable real-time carbon dioxide concentration monitoring system using smart applications. The importance of rooftop gardening in buildings is also discussed in this paper. Veranda gardening on a small scale does not immediately impact the environment, but it does improve air quality and supply adequate oxygen for many people to survive. A typical rooftop's CO 2 level is always higher than a rooftop with a garden area, according to our findings.