Humanitarian engineering projects mitigate environmental hazards disproportionately affecting health in low- and middle-income countries. However, widespread literacy deficits can create barriers in training low-literacy adults to construct these projects, indicating a need for literacy-adapted training materials. A randomized control trial in rural Guatemala tested the usability of pictorial action instructions, compared to demonstration-only methods, in training low-literacy adults (N = 60; n

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... = 30 per group) to construct a solar bottle bulb. Fourteen days after the training, participants individually constructed the solar bottle bulb. The intervention group received pictorial action instructions to guide them, and the control group completed construction based on memory. Usability was evaluated by measuring the effectiveness and efficiency of construction, as well as user satisfaction and self-efficacy levels. Effectiveness and self-efficacy were significantly better among those in the intervention group compared to the control group. Considering this, the findings support the use of pictorial action instructions in training low-literacy adults to construct humanitarian engineering projects. This method may allow more individuals in rural regions of low- and middle-income countries to successfully construct their own humanitarian engineering projects in a way that is sustainable and scalable. Further research is needed to test these instructions in different settings, on a larger scale, as well as to test the long-term effects of using pictorial action instructions.