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A new commentary suggests that the safety threshold for lead in soil should be reduced in order to better protect young children, and more testing is needed on fire-affected homes in LA area 

For Immediate Release:

Urban wildfires in Los Angeles have highlighted the increased risk of soil lead exposure, especially for young children, and the current approach to mitigating this risk in L.A. does not accurately reflect the current understanding of risks to children from lead, according to a new commentary published today in Nature from researchers at the Harvard T.H. Chan School of Public Health and the Center on the Developing Child at Harvard University.  

The LA fires in January 2025 devastated neighborhoods and left potential environmental hazards on burn sites as well as unaffected homes. “Urban fires burned through structures and materials containing lead, such as paint and plumbing, as well as arsenic and other toxic metals,” write the authors. “While the immediate aftermath of wildfires understandably centers on loss of life and property, environmental health threats posed by legacy contaminants in soil can persist for years after flames are extinguished.”  

Air quality monitors in the LA area revealed an alarming 110-fold increase in atmospheric lead levels immediately after the fires, and soil testing later confirmed elevated lead levels in areas downwind of the Eaton fire. This poses a long-term risk of exposure, particularly for biologically sensitive populations like young children, through contact with contaminated soil or inhalation of lead dust. 

”Young children’s developing biological systems are highly sensitive to influences from their developmental environment, and lead exposure at a young age poses a risk to all of these systems, particularly for brain and cognitive development,” says Lindsey Burghardt, co-author of the commentary and Chief Science Officer at the Center for the Developing Child at Harvard University. “It is critical that we implement actionable solutions to protect young children from lead exposure after a wildfire, which will have positive impacts for their health and well-being across the lifespan.” 

The commentary is recommending two critical reforms: 

1. Soil testing  (specifically “post-clearance confirmatory” testing) should be required after wildfire cleanup, as has been done for every major wildfire in California since 2007. 
2. The California’s residential Preliminary Remediation Goal (PRG) — the target threshold that residential soil should meet in order to be considered safe — for lead in soil should be lowered from 80 mg/kg to 55 mg/kg to reflect updated science and health-protective standards. 

“People all want to know the same thing, ‘Is it safe?’ The current policy was falling behind the science when it came to addressing lead in soil after the fires,” said Joseph Allen, co-author of the commentary, and Professor of Exposure Assessment Science at the Harvard T.H. Chan School of Public Health. “There are some basic steps people can take to help reduce their exposure: wash hands, don’t track in soil into your home, keep surfaces clean, upgrade indoor air filtration and use portable air cleaners.” 

Drs. Allen and Burghardt, as well as the rest of the Commentary authors, are all a part of the LA Fire HEALTH Study Consortium. The LA Fire HEALTH Study was started in response to the need for answers in the wake of the LA fires. In an unprecedented collective scientific effort to understand the short- and long-term health impacts of wildfires, researchers from Cedars-Sinai; Harvard T.H. Chan School of Public Health; the Keck School of Medicine of USC; Stanford; University of California, Davis; University of California, Irvine; University of California, Los Angeles (David Geffen School of Medicine, Fielding School of Public Health, UCLA Health); University of Texas at Austin; and Yale have launched a 10-year study of LA fires.

“Our collaborative work with the LA HEALTH consortium is unprecedented and this lead finding is one example of how scientific research can help the public in real time to protect and help the community,” said Kari Nadeau, co-author of the commentary, and Chair of the Department of Environmental Health at Harvard T.H. Chan School of Public Health.

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Read the full Commentary: Allen, Azimi, Pei, Ferguson, Burghardt, Nadeau. “Post-Fire Soil Hazards: Recommendations for Updated Soil Testing Protocols and Clearance Thresholds.” Journal of Exposure Science and Environmental Epidemiology. August 8, 2025 https://doi.org/10.1038/s41370-025-00796-w 

Read a Fact Sheet about the Recommendations here.

Media Contacts: 

Carly Stearnbourne, Harvard T.H. Chan School of Public Health  
cstearnbourne@hsph.harvard.edu, 617-571-2268 
 

Rebecca Hansen, Center on the Developing Child at Harvard University 
rebecca_hansen@harvard.edu, 617-320-2883

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A new article published Tuesday in Nature Cities finds that heat stress is responsible for productivity losses between 29.0% to 41.3% on construction job sites. As demand for new construction grows, and extreme heat affects cities across the globe, understanding the effects of heat and humidity on outdoor workers is key to protecting both worker health, as well as project timelines and costs. 

“If unmitigated, heat can lead to acute fatal outcomes (for example, heat stroke), long-term health effects or increase the risk of workplace injuries,” the study authors write. “However, even before reaching dangerous levels, physically demanding work in hot-humid conditions is associated with discomfort, irritability, loss of concentration and fatigue.” 

For businesses and employers, there is mounting evidence that shows that heat exposure leads to significant reductions in worker productivity, resulting in longer project timelines, development delays and, subsequently, substantial economic losses. 

“The conditions for construction workers can actually be a lot worse than other occupational settings, because — in addition to working outside in the elements — constructions workers often use heat-generating tools as part of the job, such as welding torches, power tools, etc. In addition, heat reflects off of concrete structures and steel, making it even hotter for the workers,” says Barrak Alahmad, study co-author, and researcher at the Harvard T.H. Chan School of Public Health. “To account for those scenarios, we measured the exact temperatures and humidity and conditions for workers at an individual level; each worker wore a small sensor around their waist that continuously, minute by minute, collected the data.” 

Research like this highlights the cost of doing nothing to mitigate heat stress among workers, and lays the groundwork for future research to investigate what types of strategies can protect workers while also boosting productivity and reducing costs for construction companies and builders. 

There are workers that are doing a lot of strenuous and difficult jobs in the hot weather,” says Dr. Alahmad. “And right now, globally, there is lack of protection for these workers.

This research also comes at a time when OSHA is in the middle of the regulatory process to establish a federal heat rule. Right now in the US, there is no federal heat rule, heat is instead governed by the general duty clause. “Creating a federal heat rule would mean that OSHA could regulate things like rest, water, and shade, and also improve planning for construction companies to try to mitigate the effects of heat ,” says Dr. Alahmad. 

This is important not only to protect workers, but also to protect the bottom line for construction projects, where loss of productivity leads to delays, missed deadlines, and cost overruns. 

The research was conducted in Taiwan where a growing demand for housing has led to a construction boom in the hot, humid climate. Co-author Shih-Chun Candice Lung from the Research Center for Environmental Changes at Academia Sinica in Taipei, Taiwan, collaborated with Dr. Alahmad on the findings. Dr. Lung is an alumnus of Harvard T.H. Chan School of Public Health, who previously studied under Dr. Jack Spengler. 

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This investigation was made possible by grant no. T42 OH008416 from the National Institute for Occupational Safety and Health (NIOSH) through Harvard–NIOSH Education and Research Center (ERC) grant. In addition, this research was supported by the Institute of Labor, Occupational Safety and Health, Ministry of Labor, Executive Yuan, Taiwan, under project nos. 1050004 and 1060047.  

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A new study clarifies the importance of nature for mental health in urban settings and provides low-cost recommendations for improving public health in cities.

As the proportion of the global population living in cities rises to 70% by 2050, mental health challenges more common in urbanites — such as anxiety and mood disorders — become even more broadly relevant. A new study from Harvard T.H. Chan School of Public Health and the Natural Capital Project (NatCap) at Stanford University shows that spending even a little time in nature provides significant benefits for a broad range of mental health conditions. The results, published today in Nature Cities, offer guidance to urban planners, policymakers, and others for how to use greenspace as a mental health solution, one that comes with additional benefits like lowering temperatures and sequestering carbon. The research team is now incorporating its findings into a modeling tool for urban planners. 

“We are working to translate what we found through this analysis to create more intuitive indicators that would be useful for decision-makers,” said Yingjie Li, postdoctoral scholar at NatCap and lead author of the study. “For example, if a city currently has 20% green space or tree cover, we can try to predict how many preventable cases of mental health disorders could be avoided if that were increased to 30%. Our software will also estimate the potential reduction in healthcare costs associated with such improvements in urban nature.”

NatCap’s flagship mapping and modeling tools, known as InVEST, are used around the world to quantify ecosystem services, or nature’s benefits to people. For years the team has been building a collection of tools focused specifically on nature in cities. 

“I was excited to be part of this work with the Stanford team. This study fills a critical gap in our ability to examine different types of nature to improve mental health outcomes. InVEST is an innovative and impactful solutions-based tool that will shape how we plan for people to enjoy healthier lives in a healthier planet” said Kari Nadeau, John Rock Professor of Climate and Population Studies, and Chair of the Department of Environmental Health at Harvard T.H. Chan School of Public Health.

The team’s analysis collates data from close to 5,900 participants across 78 field-based experimental studies, all either randomized controlled trials or pre-post intervention studies. While all types of urban nature provided benefits, the researchers found urban forests were even better for certain measures like reducing depression and anxiety. Young adults experience even greater benefits than the general population — an important data point because most mental health disorders emerge before the age of 25. Perhaps surprisingly, spending non-active, stationary time in greenspaces was more effective at reducing negative mental health outcomes like depression than active time in nature, though both are equally beneficial for positive outcomes like vitality (measured by asking how alive, alert, and energized people feel). They also found effects to be greater in Asian countries, where physiological effects may be enhanced by cultural associations with nature that “prime” people to their benefits. 

Based on the findings of this analysis, while larger city parks and forests are critical, the researchers suggest it is also important to create smaller “pocket parks” and additional street trees to increase access throughout cities. Even additional windows with views facing green spaces could be beneficial, as well as quiet, nature-filled spaces and community programming that provides passive nature exposure such as guided park meditation — relatively low-cost methods for improving public health in cities.  

At the personal level, Li has found that doing this work has improved his own lifestyle. He walks to the office more frequently and finds he is more curious about birds and plants he encounters along the way. “I also talk to my friends about thinking this way and encourage them to notice how even small moments with nature can make a difference. This work has helped me see that urban nature isn’t just good for cities, it’s good for us.” said Li.  

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Li is a postdoctoral scholar in the Department of Biology in Stanford’s School of Humanities and Sciences (H&S). Nadeau is Chair of the Department of Environmental Health and Professor at the Harvard T.H. Chan School of Public Health and Harvard Medical School. Additional co-authors on the paper are: Lisa Mandle (NatCap/Woods Institute), Anders Rydström (NatCap/Biology, H&S), Tong Wu (NatCap/Woods Institute), Yougeng Lu (NatCap/Biology, H&S), and Gretchen Daily (NatCap/Biology, H&S and Woods Institute; Yuanyuan Mao and Roy P. Remme from the Institute of Environmental Sciences at Leiden University; Xin Lan from the Department of Geography, Environment, and Spatial Sciences at Michigan State University; Chao Song from the College of Ecology at Lanzhou University;; and Andreas Meyer-Lindenberg from the Department of Psychiatry and Psychotherapy, Central Institute of Mental Health and Medical Faculty Mannheim/University of Heidelberg.

This research is supported by grants from the Stanford Woods Institute for the Environment’s Realizing Environmental Innovation Program, the Cyrus Tang Foundation, the Marcus and Marianne Wallenberg Foundation, the Heinz Foundation, the Winslow Foundation, and individual contributors John Miller and Kristy Hsiao.

Media contacts:

Elana Kimbrell, elanak@stanford.edu

Carly Stearnbourne, cstearnbourne@hsph.harvard.edu

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Earlier this month, Harvard Chan C-CHANGE and Putney Student Travel hosted 50 high school students at the Harvard Chan campus for the fifth annual Youth Summit on Climate, Equity, and Health.

The summit brought together a diverse group of students, with representation from ten states as well as students from China, Singapore, Taiwan, and the United Kingdom. Over the last five years, we have welcomed 435 students from 35 different states and 14 countries.

Students heard presentations from C-CHANGE director and deputy director Mary Rice and Amruta Nori-Sarma, as well as core faculty members, Vanessa Kerry, Gaurab Basu, and Lindsey Burghardt. Additional Harvard affiliates included K. “Vish” Viswanath, Sappho Gilbert, Barrak Alahmad, and Harvard iLab’s Rebekah Emmanuel and Virginia Canestraight of SEAS. Students also heard from policymakers such as Catherine McCandless from the City of Boston, Jeffrey Sanchez and entrepreneurs like Kip Pastor, and industry experts like Phil Dahlin of J&J.

Throughout the week, they learned how to take their passion for climate solutions and turn it into action, formed lasting relationships, and came away with a better understanding of how to make an impact in their own communities. Learn more on the Youth Summit website.

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New working paper looks at why better air in classrooms matters.

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Hello again!

A lot has happened since the first issue of our Newsletter—there are now over 5,000 of you who have completed Survey/Cognitive Test 1. Plus over 3,000 of you have completed Survey/Cognitive Test 2. THANK YOU so much to all who have participated! This research could not happen without you.

Enrollment for Survey/Cognitive Test 1 is closing soon, but there is still time to participate! We will also send more invitations for Survey/Cognitive Test 2. If you have not participated yet, please keep your eyes open for our invitation. Or, if you already have one of our invitations handy and know your study ID #, please click here to our study surveys page and participate now!

We also invited about 1,000 of you to participate in Phase II of the study, which involved providing samples of blood, toenails, and household water to better understand the relationship between environmental exposures and health. These invitations were selected randomly, so please don’t feel bad if you didn’t get one.

Preliminary Results

In Your Baby Teeth

We have been hard at work studying the data you provided, including some of the teeth you gave all of those years ago. Only a small percentage of those thousands of teeth have been examined, but below are some of the results. Each dot on the figure is one of you. This is what was in your body when you were in the womb (and just after you were born)!

Lead amounts are fairly consistent both before and after birth.

Manganese is a nutrient that the body needs, but levels that are too low or too high could cause some health problems. Manganese amounts in the teeth are lower in the postnatal period (that is, following birth)—this is exactly what normally happens, as manganese is especially important as the fetus develops.

Zinc is also a critical nutrient that the body needs and you can see that levels are much higher than lead or manganese. There is a wide distribution of levels among you and, a little like with manganese, the amounts are a bit lower after birth than before.

Your Baby Teeth Compared to Other Studies

What we generally see from around the time of your birth is that metals levels are not the same for all of you. This is to be expected. We have also compared these and other metals levels to those we see in present day groups in New Hampshire and in Nigeria. This data was published in this paper:

Punshon, T., Bauer, J.A., Karagas, M.R. et al. Quantified retrospective biomonitoring of fetal and infant elemental exposure using LA-ICP-MS analysis of deciduous dentin in three contrasting human cohorts. J Expo Sci Environ Epidemiol 34, 1000–1011 (2024).

https://www.nature.com/articles/s41370-024-00652-3

The paper asked the question: Can the use of “absolute quantification” (which means making raw data easier to compare) allow accurate comparisons between different studies? The answer was: yes!  This is an important contribution because if researchers use this approach, it makes comparisons across studies much more reliable and enriches the scientific value of the data.

The comparison for lead between the three populations is shown below. The teeth from the SLBT study showed, unfortunately but unsurprisingly, much higher lead levels due to many factors of the time period when they were collected (for example: leaded gasoline and lead paint were common, water safety and food safety effort were uncommon, and mining practices were less concerned with environmental impact).

In Your Cognitive Data

We also have data on the cognitive tests you’ve been completing. Just as a quick look we combined all your test score into one summary score. Here is a look at the distribution of that score in four different age groups. What you can see is that the distribution of scores shifts a little lower in the older age groups—this is normal with aging!  But there is still a wide variation and a lot of overlap!

Since the analysis of tooth metal takes a lot of time, we have also continued to look at your survey results. We published the following new report this year:

Roberts AL, Qiu X, McAlaine KA, Germine LT, Rotem RS, Weisskopf MG. Early-life cognitively stimulating activities and late-life cognitive function in the St. Louis Baby Tooth Later Life Health Study. Scientific Reports. 2025/01/15 2025;15(1):2105.

https://www.nature.com/articles/s41598-024-79083-x

This paper looked at how activities that engaged the brain during childhood—like playing board games, reading, writing, and doing homework—might benefit cognitive abilities (which means the mental process involved in knowing, learning, and understanding things)later in life. When we combined the cognitive results across all the tests into one measure, the results showed that study members who engaged more in cognitively-stimulating activities as children generally had better overall cognitive performance in later life. Interestingly, when we looked more closely at the age that seemed to matter most for these benefits, only activities at age 18 had a significant link to better cognitive health in older age (see figure below). Also, the benefits were most noticeable for those who had fewer of these activities during their late teens – the difference between doing no activities and doing a few activities was associated with much better cognitive outcomes, while there was a smaller difference between doing a high vs. medium amount of activities.

Our study suggests that encouraging mentally stimulating activities, especially during the late teen years, could be beneficial for long-term cognitive health, especially for teens who may have limited access to such activities.

Figure. This figure shows the relationship between cognitively stimulating activities at age 18 and cognitive scores in later life. The blue line represents the estimated relationship with cognitive scores, and the grey band around it shows how uncertain that estimate is— a wider band means less certainty. Generally, people with fewer activities at age 18 (a score of 1 or 2) had worse cognitive functioning in later life, although these differences were most clear at the lower levels of activities. The cognitive scores are standardized, meaning the average score among everyone at 0, and scores worse than that are negative and scores better than that are positive.

Thank you again for participating!

SLBT In the News/In the Community

The National Endowment for the Humanities awarded its special prize for “Untold Stories in History” to Missouri high school student Rohan Deshpande. His 2024 NHD (National History Day) project was a documentary on the St. Louis Baby Tooth Survey.

Watch the documentary

Read about the award

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Fox2, the St. Louis TV station, ran a story on the tooth study. Their website, Fox2Now, also ran an article called, “The lasting impact of the baby tooth project in St. Louis”

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Marc Weisskopf, head of the SLBT study, gave a talk called “Exploring early origins of cognitive decline: The St. Louis Baby Teeth Study” at the University of Michigan Center on Lifestage Environmental Exposures and Disease (M-LEEaD) Integrated Health Sciences Core on March 23, 2023.

What’s Next

As we mentioned above, enrollment is still underway for Survey/Cognitive Test 1 and Survey/Cognitive Test 2. Please keep your eyes open for our email and U.S. mail invitations.

We want to keep you updated on our progress through these newsletters. Plus, it gives us another chance to say thanks for your important contributions to this research! In future newsletters, we will share further details from published results, highlight key researchers, and let you know what happens next.

Many of your questions will be answered by going to hsph.me/slbt and selecting “FAQ.” But if you want to contact the study team, you can reach us at slbt@hsph.harvard.edu (fastest response time) or 617-432-0041 with any questions.

Our sincere thanks,
SLBT Study Team
Dr. Marc G. Weisskopf

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As a researcher who was originally focused on infectious disease transmission models, Parham Azizi expected he would spend his career in a lab, but, as he told Inside Climate News in a recent interview, a wave of public health crises changed all of that: first the 2019 measles outbreak in Brooklyn, the COVID-19 pandemic, Hurricanes Ida and Ian, wildfires in Maui, and most recently the January 2025 wildfires in Los Angeles, CA.

All of a sudden, all of these crises happened, and they were all related to indoor air quality,” Azimi said. “It’s kind of defined my research.

When the fires broke out in Southern California earlier this year, researchers from Harvard Chan jumped into action and formed a consortium with other research institutions at UCLA, USC, UC Davis, and UT Austin, among others. The scale of this partnership is unprecedented, but it felt like a natural step to researchers at the Harvard T.H. Chan School of Public Health.

“In these national emergencies, I think all of us in this field feel compelled to do what we can to help,” said Joseph Allen of the rapid response of public health researchers to the fires. Dr. Allen is the director of the Harvard Healthy Buildings Program, and a Professor of Exposure Assessment Science in the Department of Environmental Health. He is also helping to lead the environmental exposures team on the LA Fire HEALTH Study.

In these national emergencies,” said Dr. Allen, “I think all of us in this field feel compelled to do what we can to help.

As part of the LA Fire HEALTH Study consortium, Dr. Allen and Dr. Azizi are working to understand what type of chemicals and toxins are present in homes and neighborhoods in the aftermath of these fires.

In addition to the reports sent back to homeowners taking part in the study, the preliminary findings have also been shared in the form of Data Briefs for the general public, distributed through the study’s website. These Data Briefs have shared indoor and outdoor air quality in the burn zones, VOC levels inside of homes, and most recently, the results of the tap water testing done by Dr. Azizi and his colleagues.

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Read more about the study in Inside Climate News here: After the LA Fires, Scientists Study the Toxic Hazards Left Behind.

Read more about the LA Fire HEALTH Study Consortium here: Long-term, multi-institutional study on health impacts of Los Angeles wildfires launched.

Visit the LA Fire HEALTH Study website.

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