摘要：Aflatoxins are poisons that occur naturally in the environment. They are produced mainly by the Aspergillus flavus and Aspergillus parasiticus fungi, which can affect many of important staple crops: maize, sorghum, millet, rice, oilseeds, spices, groundnuts, tree nuts, and cassava. Hot, humid, and drought-prone climates located within 40ºN and 40ºS latitude are favorable environments for the fungus, implying that aflatoxins are most prominent in developing countries. The presence of aflatoxins in staple crops can only be confirmed through specialized testing and it is not possible to completely ‘neutralize’ aflatoxins by, for example, washing or heating. The risk of aflatoxin contamination creates a negative nexus between agriculture production and public health, because any level of aflatoxin in food makes it unsafe to consume. Chronic exposure to the B1 form of aflatoxins causes liver cancer (IARC, 2002), and is linked to cirrhosis of the liver (Kuniholm et al., 2008) as well as to immune suppression in humans (Williams et al., 2004). Evidence also suggests that aflatoxins may cause stunting in children (Khlangwiset et al., 2011). Though this issue is recognized by the global markets, in many developing countries aflatoxin-contaminated food produced by the agriculture sector is consumed domestically, resulting in significant social costs of diet-related illness. This represents a failure of private markets to generate a socially efficient quantity of the ‘food safety’–a public good. Even when domestic food markets can discern the aflatoxin-free food, ‘food safety’ good may be under-provided. This is because the contaminated food could still be consumed by poorer households that are unable to participate in the aflatoxin-free food markets. To assist potential government interventions in the context of sub-Saharan Africa, we developed a conceptual framework for assessing the interrelated public health, trade, and agriculture impacts of aflatoxin contamination in maize (a key African staple) and groundnuts. We propose an integrated approach to assess the relative impacts on these sectors. The relative importance of these impact categories depends on the final uses of the susceptible crops. It also depends on the level of public awareness and the effectiveness government food safety standards, both of which are low in sub-Saharan Africa. Potentially contaminated crops are traded locally, used for own consumption or as animal feed. However, they do not experience revenue losses from periodic aflatoxin outbreaks nor do their production costs reflect the use aflatoxin controls. Short-run international trade losses are also low, because the food security concerns limit export. Consequently, the impact of aflatoxin outbreaks is predominantly on the public health. To evaluate the public health impacts, we conducted the risk assessment and developed a model for the risk characterization. Like other studies on aflatoxin health impacts (e.g., Liu and Wu, 2010), we concluded that only the excess incidence of liver cancer can be quantified, taking into account the susceptible hepatitis B-positive sub-population. We monetize the public health damage using a Value of Statistical Life (VSL) transfer approach (Hammitt and Robinson, 2011). The health impacts model includes an explicit characterization of uncertainty from several sources. We quantified the liver cancer impacts resulting from consumption of aflatoxin-contaminated maize and groundnuts in Nigeria and Tanzania. Based on the published aflatoxin sampling results for Nigeria, we estimated that a consumption-weighted aflatoxin B1 contamination level could be 67ppb. While Nigeria does not have an aflatoxin B1 standard, its safety standard for total aflatoxin in food for human consumption is 4ppb. Our contamination level estimate is highly uncertain. However, chronic exposure to aflatoxin B1 at 67ppb could be causing as many as 7,761 liver cancer cases per year out of the estimated 10,130 total liver cancer cases in Nigeria in 2010. Exposure to this level of contamination was estimated to result in monetized damages between $380 and $3,174 million (in 2010 U.S. dollars). It is noteworthy that the high monetary estimate at 67ppb constitutes roughly 1.6% of Nigerian GDP in 2010, which was $197 billion 2010 U.S. dollars. In Tanzania, the prevalence of aflatoxins is lower than in Nigeria. However, exposure to aflatoxins could still be significant because of high maize consumption (400-500 grams per day on average). At 5ppb, which is the national safety standard for aflatoxin B1 in maize and groundnuts, the total estimated annual excess liver cancer cases were 546, accounting for more than a third of the total estimated liver cancer cases in Tanzania in 2010. The monetized health impact at the 5ppb aflatoxin B1 contamination level is between $18 million and $147 million (in 2010 U.S. dollars). Because liver cancer risk from aflatoxin B1 exposure also depends on the hepatitis B prevalence, focusing efforts on immunization programs could generate a three-fold reduction in the number of aflatoxin-caused liver cancers cases.
