Exposure refers to people, infrastructure, property, housing, production capacities, and other tangible human assets located in a geographical area likely to be affected by hazards. Disaster impact is a direct function of exposure, and if a hazard occurs in an area having no exposure, then there is no risk.
In October 2013 a Category 5 super typhoon Lekima hit the northwest Pacific Ocean. Even though the wind velocity reached around 240 km/h there was no impact on people or assets. In November 2013 another Category 5 super typhoon Haiyan (Yolanda) hit the region, with wind velocity reaching 315 km/h. Haiyan affected 11 million people, causing more than 6000 casualties, and economic loss of more than US $ 1.5 billion. The difference in impact was because there were no people or property in the path of Lekima; in other words, there was no exposure.
The extent to which exposed people or economic assets are actually at risk is generally a function of their vulnerability, as it is possible to be exposed but not vulnerable. A house constructed using modern precepts of earthquake safe construction though exposed to seismic hazard by virtue of its location is more vulnerable.
However, in case of extreme hazards the degree of disaster risk is dependent on exposure more than vulnerability. As in case of a Great Earthquake (Magnitude > 8.) all exposed structures, whatever be the technology used in construction, face high risk. Likewise in case of the December 26, 2004 Indian Ocean Tsunami all those exposed to tsunami were at risk, irrespective of their income, ethnicity or social class.
Drivers of exposure
Driven by population growth, migration, urbanization, and economic development people, and economic assets often tend to get concentrated in areas exposed to hazards. Previous disaster incidences in an area can also drive exposure by forcing people from their lands, and to increasingly unsafe areas.
Exposure is thus variable, and changes over time, and from place to place.
Many hazard prone areas, such as coastlines, volcanic slopes, and flood plains sometimes attract economic, and urban development as these offer significant economic benefits, or are of cultural or religious significance to the people who live there. As more people, and assets are exposed, risk in these areas gets more concentrated. At the same time, risk also spreads as cities expand, and as economic, and urban development transform previously sparsely populated areas.
Large volumes of capital thus continue to flow into hazard-prone areas, leading to significant increases in the value of exposed economic assets. If global exposure continues to trend upwards, it may increase disaster risk to dangerous levels.
Exposure assessment
Measures of exposure can include the number of people or type of assets in an area. These can be combined with specific vulnerability, and capacity of the exposed elements to any particular hazard to estimate the quantitative risks associated with that hazard in the area of interest.
Exposure modelling plays a critical role in risk assessment. The information to be utilised for developing exposure data sets can be derived from various sources, and methods. Resolution of the data sets refers to how thoroughly defined the data are; for instance, exposure data over a large geographical area (low resolution) may hide the true picture at the local level.
Common exposure data sources include household, and census survey data, aerial photographs, and satellite imageries, and individual architectural and structural drawings. Crowdsourcing that refers to obtaining information or input into a task by enlisting the services of a large number of people typically via the internet, is becoming a common, and valuable tool for collecting local level data, as well as validating regional data, but this approach is limited by the type, and quality of data.
Comprehensive geospatial inventories that include public infrastructure, assets, and investment provide a sound basis for assessing exposure level but these are often not available publicly. So there is a pressing need to develop these inventories. State-based agencies, statistical offices, census data, investment and business listings, employment figures, and existing geographic information system (GIS) data are common sources of exposure information at a regional scale, and above.
At the global scale efforts to generate globally consistent exposure data sets in terms of the quality, and resolution have increased. Methodologies however need to consider dynamic nature of exposure because it evolves over time as a result of urbanisation, demographic changes, modifications in building practices, and other factors. At the lowest (coarsest) level of resolution, national statistical agencies, census data, global databases, and remote sensing are used for developing exposure data.
How do we reduce exposure?
Economic exposure in high-hazard areas is trending upwards, and if not checked or reversed disaster risk is set to increase. We therefore need to act now to reduce exposure, and build capacity, and resilience in these areas of growing exposure.
When it is not possible to avoid exposure to events, land use planning, and location related decisions should be accompanied by other structural or non-structural measures for preventing or mitigating risk. Diversification of economic assets is one such measures that besides reducing exposure ensures balanced growth, and development.
For instance, in the case of the December 26, 2004 Indian Ocean Tsunami the only possible strategy to save lives would have been to reduce exposure through timely evacuation, which depends on the existence of reliable early warning systems, and effective preparedness planning, and then to compensate for loss through insurance of other risk financing instruments.