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LifeStraw

LifeStraw Family is a high-volume point-of-use water filter for use in homes. The device, which uses gravity to force water through narrow fibers, does not require batteries or electricity and removes bacteria, parasites, viruses, and particles from water. Water enters into the filter where fibers trap contaminants, allowing the newly cleaned water to exit through the tiny pores of the fibers. LifeStraw is cleaned via backwash and can filter up to 18,000 liters of water (enough for a family of five for three years).

Unit of interventionone filter

Volume of water treated18,000 L

Target populationAll ages

Model Overview

PSI’s LifeStraw impact model is based on the Lives Saved Tool (LiST). LiST is used to estimate the number of deaths averted with complete coverage of improved water in select countries. This number of deaths averted at the population level is translated to deaths averted per LifeStraw, using parameters such as existing coverage of water treatment products and access to clean water, per capita usage of water per day, and liters of water treated per LifeStraw. Once we have deaths averted per LifeStraw, we apply data from the 2010 Global Burden of Disease study to estimate the corresponding number of DALYs averted.

Model Outputs (impact metrics)

Estimates of DALYs averted, deaths averted, CYPs provided, and unintended pregnancies averted represent the projected health impact of the intervention. It is projected because it has not been directly measured.

Examples based on distribution of LifeStraws in Tanzania in 2015 Statement of modeled results, examples:

In 2015, an estimated 9 deaths and 631 DALYs would be averted if 10,000 LifeStraws were distributed in Tanzania.

For more details about how PSI models the impact of LifeStraws, see below.

Model Details

Step 1: Running a projection in the Lives Saved Tool (LiST)
LiST is a multi-cause mortality model developed by Johns Hopkins Bloomberg School of Public Health that estimates the number of deaths averted (or lives saved) through the scale up of maternal and child health interventions.

PSI begins by running a projection in LiST for select countries. In this projection, access to treated water is increased from the current, country-specific baseline (taken from DHS surveys) to 100%. LiST then projects the number of deaths averted among children under five by this increased level of coverage.

Step 1 Output:
Number of additional deaths averted (or lives saved) among children under five if coverage of LifeStraws increased from baseline to 100% in select countries

Step 2: Estimating deaths averted per LifeStraw among children under five
PSI uses the step 1 output (deaths averted at 100% coverage of LifeStraw) to determine the number of deaths averted by a single LifeStraw. To do this, we divide the number of deaths averted at 100% coverage by the number of LifeStraws needed to reach 100% coverage among children under five.

PSI estimates the number of LifeStraws needed to reach 100% coverage using a number of parameters, including existing coverage of water treatment products and access to clean water, per capita usage of water per day, and liters of water treated per LifeStraw. We also account for wastage of packets in the supply chain, assumed to be 10%.

Step 2 Output:
Under five deaths averted coefficient per LifeStraw

Step 3: Estimating DALYs averted per LifeStraw among children under five
A DALY (or disability adjusted life year) includes two components: years of life lost due to premature death (YLL) and years lived with disability (YLD). DALYs averted are in turn comprised of YLLs averted and YLDs averted or, put simply: death and disability that is prevented by PSI interventions.

To estimate YLLs averted per LifeStraw among children under five, PSI first determines the number of years of life lost per death among children under five due to diarrhea in select countries. This is equal to the life expectancy at the average age of death from diarrhea. Age specific life expectancy is taken from the 2010 Global Burden of Disease study. The number of years of life lost per diarrhea death is then multiplied by the number of deaths averted per LifeStraw among children under five (under five deaths averted coefficient), calculated in step 2 above for a selected country. This gives us the YLLs averted per LifeStraw.

To estimate YLDs averted per LifeStraw among children under five, we use a YLD/YLL ratio, based on the 2010 Global Burden of Disease study. This ratio represents the relative number of years lived with disability for every year lost due to death from diarrhea. We apply this ratio to the number of YLLs averted per LifeStraw to determine the number of YLDs averted per LifeStraw among children under five.

Finally, YLLs averted and YLDs averted are added together to determine the number of DALYs averted per LifeStraw among children under five.

Step 3 Output:
Under five DALYs averted coefficient per LifeStraw

Step 4: Extrapolate deaths and DALYs averted among children under five to all ages
LifeStraws impact individuals of all ages. In order to estimate impact at the population level, we have to extrapolate deaths and DALYs averted among children under five to deaths and DALYs averted among all ages. This is done by using two new ratios, both of which are calculated using the 2010 Global Burden of Disease study:

Each ratio is applied to its respective impact coefficient (deaths or DALYs averted among children under five per LifeStraw) to determine the number of deaths and DALYs averted among all ages.

Step 4 Outputs:
Deaths averted coefficient (all ages)
DALYs averted coefficient (all ages)