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Air pollution is critical to public health globally, causing around 7 million deaths annually . Its regulation can be aided by providing the public a way to monitor it, informing individuals’ daily decision-making to limit its effects, and raising awareness that may lead to broader change. This project achieves this task with a budget-focused and compact all-in-one unit that provides remote telemetry data for multiple air pollutants that are impactful on individual health. All data is uploaded via mobile uplink and displayed in near real-time on a user-friendly website, with any levels above a caution threshold highlighted. During testing, accurate data about the levels of identified pollutants were retrieved across a week by installing two finished units in various locations around Coventry and Warwickshire, which were processed and presented in a coherent manner.
Each of the manufactured units costs a total of £1156, far below the price of comparable retail units (upwards of £5000). Pollution in test sites was found to be largely within acceptable levels, with only one pollutant being above the acceptable threshold in one area – PM10 was on average 6% above the allowable threshold of 50ppm.
Air is all around us and essential for life. The World Health Organisation has classed air pollution as ‘one of the greatest environmental risks to human health’ with 99% of the world population living in areas with insufficient air quality . In 2019, the United Nations stated that air pollution was the cause of over 7 million deaths per year . Given the impact of air quality on global health, making data about the local environment available to all could be a pivotal step in enacting societal change to proactively combat harmful emissions.
Beyond informing the general public, industries such as construction , pharmaceuticals , and the automotive sector  are all under pressure to reduce their total emissions – in many instances, to net zero. If the technology to monitor these emissions can be made affordable, portable, and adaptable enough, it could serve as a useful tool for public authorities, institutions and companies to continually monitor their performance. Furthermore, this would provide a method of assessing the impact of trial ‘green’ initiatives currently being rolled out by local councils in the UK such as electrified public transport and district heating schemes .
Several air quality monitoring systems are currently available on the market. These are primarily monitoring stations which are rare and not particularly localised. These can cost up to £1,000,000, however some lower-cost systems have been produced in recent years. One of these is the Oizom unit which allows the customer to personalise their sensors from a selection of over 30 different types of parameters that can be measured . Its primary disadvantage is its cost, quoted to the University of Warwick (with a 30% educational discount) at £3,300 .
Several academic projects aiming to develop an air quality monitoring station have also been published. One such project aims to craft an Internet-of-Things environmental monitoring solution by developing a wireless sensor network that can monitor noise, light, temperature, humidity, and CO levels of a building . Although this unit is suited for an indoor environment as its sensors are individually less obtrusive, they are not suitable for outdoor use and the sensor loadout is limited. The Smart Citizen System (SCS) is another academic project aimed at monitoring air quality . It is a modular outdoor urban environmental sensing solution consisting of a slew of custom-made circuit boards, components, and cases, with a focus on modular design and a user-friendly appearance.
The FRESH unit aims to monitor all critical air quality indicators whilst remaining both cost effective and easy to set up. Parameters have been selected based on their health impacts; these are presented in Table 1. This data will be processed and made available in near-real time in a way that can be easily understood by the public to allow them to make constructive use of the information provided .
Carbon Monoxide (CO)
Car exhaust emissions
Headaches, shortness of breath
Sulfur Dioxide (SO2)
Nitrogen Dioxide (NO2)
Throat inflammation, coughing
Carbon Dioxide (CO2)
Headaches, dizziness, tingling
Total Volatile Organic Compounds (TVOCs)
Eye irritation, nausea, headaches
Particulate Matter (PM)
Diesel engines, Wood fires
Lung & heart damage
Car exhaust fumes
Asthma, chronic bronchitis
Ultraviolet (UV) Radiation
Skin cancer, cataracts
Table 1: Air quality indicators and examples of their effects on human health
To measure the pollutants previously listed, the FRESH unit employs a variety of sensors selected for their accuracy and reliability. When selecting these sensors, the most important consideration is to ensure they can detect concentrations within the ranges classified by the UK and European Union as acceptable pollutant levels . Cost, quality, durability, availability, interface and sensing technology are also considered, leading to the range of sensors seen in Table 2. Most sensors used do not require any calibrations after being received from the manufacturer. The electrochemical sensors were calibrated against the Oizom unit as this was a reliable source of data.
Nondispersive infrared (NDIR)
Optical (laser scattering)
Odour (VOC Subset)
Electret microphone on amplifier board
Table 2: Selected sensors and their description
The sensors are organised within the unit in terms of technology used as well as internal location. Some of the sensors can take reliable readings from within an air cycling chamber (Section 7.3) whereas others need to be exposed to the outdoor environment (Section 7.2). To optimise the design following these criteria, the MOX sensors are grouped onto a custom-made PCB. The CO2 sensor is daisy chained onto this because both sensor boards need to be placed within the air chamber. The electrochemical sensors are grouped together in the chamber on a pre-made 4-way Alphasense analogue front end (AFE) board.
This board simplified the design process as the analogue output of the electrochemical sensors would have required a complicated support circuit. The outdoor sensors are on individual breakout boards to enable easy positioning within the unit. The PM sensor is standalone, with a fan built in, so does not require a breakout board or PCB. To collect and organise data coming from all the sensors, a custom-made main PCB was designed. This PCB contains the microcontroller which interfaces with all the sensors as well as the fans and the GSM (Global System for Mobile communication) module which transfers data to a server in real time.
Finally, a power system is implemented, consisting of a battery charged through a solar panel via a DC-to-DC converter. Power to the unit is regulated via a latching RGB switch. The part number and manufacturer for all the additional components used in the design can be seen in Table 3 and an overall system diagram can be seen in Fig 1.
5V DC Axial Fans
Adafruit FONA 3G GSM
12V, 10W Solar Panel
5V, 47Wh Battery
12-36V to 5V DC-DC Converter
Traco Power TMDC202411
Latching RGB Button
Table 3: Other components and their part numbers
As mentioned in section 5, the unit required MOX sensors and electrochemical sensors to be placed within an air chamber as well as outdoor sensors and a PM sensor. To organise these sensors, two custom made PCBs were designed. The first circuit board was referred to as the MOX board as it contains metal oxide technology sensors (TVOC and odour sensor) as well as a temperature and humidity sensor. Unlike the TVOC and temperature/humidity sensor which communicate via inter-integrated circuit (I2C), the odour sensor has an analogue output and requires separate heater circuits to set a precise temperature for each gas measured and enable accurate readings. These circuits are integrated into the MOX board design.
The main board is designed to receive and collect data from all the sensors and therefore holds a variety of connectors including custom-made connectors for the AFE board and PM sensor. To process this data the ATMEL SAMD51 microcontroller was chosen as its large processing power and RAM makes it capable of dealing with the large selection of sensors. Furthermore, this board could be programmed in C using the Arduino IDE which allowed the use of custom-made libraries for all the selected digital sensors. The analogue sensors (AFE board, microphone, and odour sensor) require an ADC to record their output – an 18-bit ADC was selected as its high resolution enables accurate translation of the readings. Additionally, the main board features a real time clock (RTC) powered by a separate battery which enables time to be recorded accurately even when the board is powered down. An SD card slot was also added to save data in the event of a GSM malfunction.
The microcontroller is programmed to collect data from each sensor using pre-made Arduino libraries. This data is then sent to a server to be organised and transferred to a database and a website. To minimise power usage, data readings are only taken once every ten minutes and before readings are taken, the fans are turned on to renew the air within the chamber and ensure that the readings reflect the outside world conditions. The length of time during which the fans are on was based on the fan discharge of 6.8 m3/h as well as the air chamber volume of 0.001 m3 from which it was calculated that the fans needed to be on for 0.5s to renew the air within the chamber. To allow for fluctuations from the theoretical flow rate this value was multiplied by ten and the fans were left on for 5s. Once the fans are turned off, the sensor readings are taken which ensure that the noise of the fans does not interfere with the microphone readings.
As shown in Fig 2, data collected from the sensors is formatted as a message to be sent through GSM and displayed on a website. The data is also written to an SD card as a backup in the event of an error occurring with the server.
Case selection formed a significant part of the early stages of the project as the case must fulfil certain criteria to ensure correct functioning of the unit. Specifications like electromagnetic shielding are desirable to prevent external signals from interfering with internal components, as is Ingress Protection of at least IP54. Inspection of similar devices provided an estimation of dimensions for the ideal unit: rectangular, and approximately 250 x 100 x 180mm. The concept of retrofitting an outdoor solar floodlight case was explored as a way to make assembly and construction easier, but ultimately the Schneider NSYDBN2520 Electric Spacial SDB Steel Wall Box of dimensions 93 x 256 x 206mm (Fig 3) was chosen.
Some sensors require external mounting so that they are exposed to the ambient environment, but each has specific requirements for protection and reading accuracy. Table 4 details these requirements, and the procedure taken to overcome them.
Rain/dust ingress, impact, exposure to sun
Mounted on top and protected by a 35mm diameter UV- permeable glass dome.
Rain ingress, thermal retention inside the chamber
Mounted in an insulated PMMA housing at the bottom of the case, with rubber grommet to disrupt the path of dripping water.
Rain trickling, air pressure inside chamber
Glued to the outside of the chamber, with rubber sealing applied to the external slot to disrupt rainwater
Mounted at the bottom of the case and surrounded by a rubber gromet to disrupt the path of dripping water.
Rain, dust, insects
Bolted onto the base with sanding mesh placed in front.
Rain ingress, electrical insulation
Mounted on top with hole enclosed by a waterproof rubber gromet.
Choice of a splashproof button mounted to the base.
Solar panel connector
Plastic cable gland mounted on the side.
The AFE board, MOX board and CO2 sensor need to be in contact with outside air to provide accurate readings. To enable this whilst simultaneously protecting them from rainfall and humidity an air chamber was made from poly(methyl methacrylate) (PMMA). This chamber was sealed from the rest of the unit and a fan was placed on either side, propelling air in opposite directions to encourage a cyclical flow through the chamber.
In the initial design, a C-shaped chamber with a fan at each end is used to create a directed airflow within the chamber. Each part of the chamber was to be manufactured separately and then glued together and into the case, to fully seal the chamber and prevent accidental travelling of items during unit movement. Consideration of assembly and removal/reinstallation of components resulted in a realisation of the benefits of a removable chamber. Therefore, the concept of gluing the chamber to the case has been replaced by standoffs to screw the chamber down, while rubber is applied along the chamber edges for sealing purposes. To further simplify chamber construction, the AFE and MOX boards have been adjoined to allow the C-shaped inner wall to be removed, resulting in a monolithic chamber formed from one folded piece of PMMA. For all components mounted to the base of the unit, cut-out notches are included for wired connections. Moving the sensors closer to the fans ensured a suitable airflow over them while freeing up space in the unit. The final CAD design is shown in Fig. 3a, while a manufactured unit is shown in Fig. 3b and 3c.
The unit was tested on the University of Warwick campus, Coventry, United Kingdom in areas with a high variation in population density or traffic volume, to see how this affects the sensor readings. It was also placed in the same location as the commercial unit to allow comparison with the Fresh unit data for calibration of its sensors. Fig. 4 (a) shows the results from the Oizom unit and the Fresh unit for PM10 readings. As can be seen, the readings are very similar indicating the accuracy of the Fresh unit. There is a slight delay in readings from the Oizom unit due to it wirelessly transmitting the data and the readings are smoother as only one result is recorded per hour.
The average for each chemical pollutant measured was calculated for a 24-hour period and compared to the harmful thresholds given by the WHO regulations and medium pollution levels set for at-risks groups .
The results collected over the week of testing are presented in Fig 5 and Table 5 in which the locations were classified as red, yellow or green based on the air quality measured. If one of the pollutants measured exceed the first harmful threshold, the entire area is considered to have very poor air quality and was shown in red. If the medium pollution threshold is reached, the area is considered to have a poor air quality and is shown in yellow. If neither threshold is reached the air quality is considered good and the area is shown in green.
Table 5: Average air quality levels
To obtain an accurate measurement of the power consumption of the unit, a USB voltage current detector was used. Using a systematic approach, the power consumption of each sensor or board was measured as well as the overall energy consumption which was 0.96Wh. When active and taking readings the power consumption of the unit was 1.65W and when idle it decreased to 950mW. Based on these measurements it was established that the MOX board consumed the most power (497mW) with all other sensors consuming 150mW or less. Implementing power saving measures such as removing power to the MOX board when not taking readings could result in a 53% decrease in power consumption when idle. As the unit is idle 99% of the time this would result in significant power savings enabling the unit to last long enough for the solar panel to fully recharge the battery.
In this project, two units were successfully designed, manufactured, and tested around Warwickshire. These units returned reliable data comparable to a similar retail unit, whilst being manufactured for a fraction of the cost - £1150 versus upwards of £5000. Air quality was tested in various locations considered to have high levels of pollution and was found to be generally acceptable – pollutants were typically below acceptable thresholds, with the exception of PM10 which was found to be 2.965ppm above the threshold of 50ppm, on the
Piazza at the University of Warwick campus. Although it was not possible to send the data collected over GSM due to time constraints, it was successfully presented in a way that can be easily understood by the general public.
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