individuals who purchased a test at the same price of BDT45 were also given a metal placard of a color depending on the arsenic level: blue for arsenic up to 10 ppb (parts per billion or micro-grams per liter), the World Health Organization guideline for arsenic in drinking water, green if above 10 and up to 50 ppb, and red if ‘unsafe’, that is, above the national government standard of 50 ppb. Similar metal placards have been used before in some testing campaigns (Opar et al. 2007, van Geen et al. 2014), as a more durable alternative to the routine strategy—adopted also during past nationwide testing campaign in Bangladesh—of applying to the well spout red or green paint that often becomes invisible within a year (Pfaff et al. 2017). Such visible indicators are a reminder about the status of specific tube wells with respect to arsenic and can spread this information throughout the village. In different contexts, other researchers have found large impacts of reminders on health-related behavior, for instance through the use of SMS messages, see Pop-Eleches et al. (2011) and Raifman et al. (2014). However, the cost of the placards (about BDT80) is high enough to increase significantly the total cost of testing. It was therefore important to determine whether the placards made any difference relative to the alternative solution (adopted in the two experimental arms described earlier) of informing the household with a less expensive laminated card indicating the test result and encouraging the household to keep the card in the house.

Despite much progress on numerous health indicators (Chowdhury et al. 2013), Bangladesh remains in the midst of a severe health crisis due to the widespread presence of naturally occurring arsenic (As) in shallow aquifers (see Ahmed et al. 2006, Johnston et al. 2014, and Pfaff et al. 2017). The problem, due to the widespread presence in the country of geological conditions conducive to accumulation of arsenic in groundwater, is compounded by millions of households in rural areas relying on water from privately owned, unregulated shallow tube wells for drinking and cooking. Using nationwide data from 2009, Flanagan et al. (2012) estimated that, in a country of more than 150 million people, about 20 million were likely exposed to arsenic levels above the official Bangladesh standard of 50 ppb, while almost one third of the population was likely exposed to levels above the significantly lower WHO guideline of 10 ppb.

The most visible health consequences of chronic exposure to arsenic from drinking tube well water in South Asia, such as cancerous skin lesions and loss of limb, were recognized in the state of West Bengal, India in the mid-1980s (Smith et al. 2000). It has since then been shown on the basis of longterm studies in neighboring Bangladesh that arsenic exposure increases mortality due to cardiovascular disease, and may inhibit intellectual development in children and be detrimental for mental health (Wasserman et al. 2007, Argos et al. 2010, Rahman et al. 2010, Chen et al. 2011, Chowdhury et al. 2016). These health effects are accompanied by significant economic impacts: exposure to arsenic has been estimated to reduce household labor supply by 8% (Carson et al. 2011) and household income by 9% per every earner exposed (Pitt et al. 2015), while Flanagan et al. (2012) calculated that a predicted arsenic-related mortality rate of 1 in every 18 adult deaths represents an additional economic burden of USD13 billion in lost productivity alone over the next 20 years.

Piped water from regulated and monitored supplies would likely be the most effective policy answer, but such a solution would require immense investments in infrastructure that may not be sustainable 3