# A Survey of Wireless Sensor Network Based Air Pollution Monitoring Systems
## 2 Air Quality Standards  
Pollutants are emitted by human activities and natural sources. Hundreds of hazardous pollutants in our living environment have been identified [22]. However, six of these pollutants are well studied and ubiquitous in our daily lives, including carbon monoxide (CO), nitrogen dioxide (NO\({}_{2}\)), ground level ozone (O\({}_{3}\)), sulfur dioxide (SO\({}_{2}\)), particulate matter (PM) and lead (Pb) [23]. The health effects (see Table 2) and environmental effects caused by these pollutants can be found in [24; 25; 26; 27; 28]. Governments and organizations have put regulation limits on these pollutants to reduce the risks. The United States Environmental Protection Agency (EPA), the World Health Organization (WHO), the European Commission (EC), the Chinese Ministry of Environmental Protection (MEP) and the Environmental Protecting Department (EPD) of Hong Kong have declared different standard limits for these pollutants (see Table 3). In order to help the public understand the current air quality easily, the government and organization agencies introduced an indicator called Air Quality Index (AQI). AQI measures the \"condition or state of each relative to the requirements of one or more biotic species and/or to any human need or purpose\" [30]. In a word, it tells the public how \"good\" the current air quality is or the forecast air quality will be. Different agencies may use different air quality indices [31, 32, 33, 34]. To illustrate the concept of AQI, an AQI example introduced by the Environmental Protection Department (EPD) of Hong Kong [35] called Air Quality Health Index (AQHI) system is given (see Table 4). The AQHI system provides a better understanding on health risks to the public and suggests detail precautionary actions with respect to each AHQI level [36]. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**Pollutant** & **Health Effects** \\ \hline Carbon Monoxide (CO) & Reducing oxygen capacity of the blood cells leads to reducing oxygen delivery to the body’s organs and tissues. Extremely high level can cause death. \\ \hline Nitrogen Dioxide (NO\({}_{2}\)) & High risk factor of emphysema, asthma and bronchitis diseases. Aggravate existing heart disease and increase premature death. \\ \hline Ozone (O\({}_{3}\)) & Trigger chest pain, coughing, throat irritation and congestion. Worsen bronchitis, emphysema and asthma. \\ \hline Sulfur Dioxide (SO\({}_{2}\)) & High risk factor of bronchoconstrtion and increase asthma symptoms. \\ \hline \multirow{3}{*}{Particulate Matter (PM\({}_{2.5}\) \& PM\({}_{10}\))} & Cause premature death in people with heart and lung diseases. Aggravate asthma, decrease lung function and increase respiratory symptoms like coughing and difficulty breathing. \\ \hline \multirow{3}{*}{Lead (Pb)} & Accumulate in bones and affect nervous system, kidney function, immune system, reproductive systems, developmental systems and cardiovascular system. Affect oxygen capacity of blood cells. \\ \cline{1-1}  & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The six common pollutants and their health effects.