Q&A with King Center Postdoctoral Fellow Yoshika Crider

Yoshika Crider is a postdoctoral fellow at the King Center and an interdisciplinary global health researcher with a background in environmental engineering and epidemiology. 

Crider's research interests include WASH (water, sanitation, and hygiene), global health, passive chlorination technologies, interventions against NTDs (neglected tropical diseases), and gender equity.

Your research focuses on WASH (water, sanitation, and hygiene) interventions, with an emphasis on childhood health. What brought you in this direction during your academic career?

I was an undergrad at Stanford and, after my sophomore year, I spent a summer in Dhaka, Bangladesh, as a research assistant for Prof. Jenna Davis in Civil & Environmental Engineering. I was based at the renowned Centre for Diarrhoeal Diseases Research (icddr,b), working on designing and testing technologies to automatically treat drinking water with chlorine in urban slums. At icddr,b, I met incredible colleagues with whom I still collaborate today. But initially, I was supposed to spend that summer working on sustainable agriculture in Japan! I was always interested in the environment and global health and, growing up in Kansas, farming and food access seemed to me an obvious way to link the two. When that summer program was canceled and my plans had to change, I went to Dhaka for the summer, and it completely changed the trajectory of my life. I ended up living and working there for a couple years after graduation, and that experience convinced me to pursue a PhD. In places like the slums of Dhaka, lack of access to the most basic services leads to diseases that most of never think about in the United States. For example, diarrhea is still a leading cause of death among young children globally. Children are at increased risk of some illnesses, and childhood illness can also have long-lasting consequences, so early prevention is key.

Your research in 2018 published in Science of The Total Environmentexamined the taste sensitivity of water when treated with chlorine. Aversion to the taste or smell of chlorinated water has been thought to limit use of chlorine treatment products. What were the thresholds of chlorine users were willing to accept? Were you surprised by your findings?

We found that end users could reliably taste free chlorine at 0.71 mg/L and that the taste was unacceptable above 1.25 mg/L. Chlorine is such an inexpensive water treatment option, and there are lots of products that are widely promoted for household-level treatment, most of which dose around 2 mg/L. So we found that, at least in Dhaka, the typical chlorine doses for household water treatment products are unacceptable to end users. I think I was a little surprised that the threshold wasn’t lower, because there is this idea that the taste and smell issue is too high a barrier, but, where we were working, it was great to find out that it was totally possible to effectively chlorinate water and have it still be acceptable to end users. In some places, where the untreated water is turbid or the quality is really bad, a higher dose might be required to make it safe and to ensure some protection during storage. But, at our sites in Dhaka, the untreated water was contaminated, but still high enough quality that lower doses could work, and water wasn’t stored as long as it is in some other places. And at the end of the day, it doesn’t matter if the water is technically safe if no one will drink it. We used those results to inform the implementation strategy in a subsequent cluster randomized controlled trial of a passive chlorination technology. Even at lower doses, we saw a significant reduction in child diarrhea.

One great thing is that the data has been used for other analyses too. For example, Daniel Smith (Stanford PhD alum in Jenna’s lab), used the data to model how people might actually be exposed to more contamination when water is treated with higher levels of chlorine, because they’ll choose to use alternative and lower quality water sources instead of one that has a chlorine taste or smell that they find unacceptable. It illustrates how important it is to consider end user preferences.

You published a paper in 2021 in The Lancet Global Health about how materials used in household floors may affect the transmission of helminths, or parasitic worms. The study found that households in Bangladesh and Kenya with finished floors, such as those made out of concrete, had lower prevalence of intestinal worms and parasites. What prompted you to examine the floors of households?

This is a topic I’m really excited about! In WASH, there are a lot of interventions that require behavior changes. For example, the main strategy for low-cost drinking water treatment is to provide poor households with tools to treat their own water every day. If they forget or are too busy just once, they could get sick. But changing a household floor from dirt to concrete is a one-time upgrade that could interrupt the transmission of soil transmitted helminths, which require soil for a part of their life cycle. The paper was led by Jade Benjamin-Chung, an assistant professor in Population Health Sciences and a King Center faculty affiliate, and we used data from a huge randomized controlled trial of WASH interventions to answer a new question about benefits of improving household floors, motivated by some prior research. Right now, I’m working with Jade, Marie (a King Center RA), and Christlee (a King Center predoc) and leading a meta-analysis of that association between household finished flooring and child diarrhea and gut health. We’re compiling individual level data from many different trials to be able to answer a lot of interesting questions, like how this relationship might change if there are animals around. Accessing and compiling all these datasets has been a big task, and I wouldn’t be able to do it without King Center research assistant support.

You have been at the King Center as a postdoctoral scholar for almost a year now. What has your experience been like at Stanford and the King Center?

It’s been a wonderful experience, and I’m incredibly grateful for the support I’ve received from the King Center. To have the intellectual freedom AND the resources to be able to pursue my research is the dream! I did my PhD in an interdisciplinary department, and I’m happy to be in an interdisciplinary community here. At a recent talk, I heard an epidemiologist say, “Health inequities stem from resource inequities,” a succinct statement of this connection I see between my work on safe water and poverty-related diseases and my economist colleagues’ work on resource distribution. The other postdocs here work on very different topics, but I really appreciate what I’m learning from this community.