The Research Goal.
Very little research has analyzed the time it takes for UV-C lamps to fully emit the 1J/cm^2 needed to decontaminate N-95 masks for reuse. After verifying UV-C light’s suitability and box material/layout compatibility, our goal was to find the maximum, minimum, and average time for low cost, publicly available 8 Watt T5 tube UV-C lamps to reach the necessary irradiation level to decontaminate a N95 mask. With this data, those creating similar devices can utilize the field procedure to quickly calibrate using a minimal number of dosimeters and decrease the overall project cost.
Our Research, Your Project.
This investigation was conducted in response to the N95 mask shortage during the Covid-19 pandemic, which has been amplified in remote and under-served areas. This research could be applied to other events that cause PPE shortages, and our methods are widely applicable to other diseases with the correct germicidal irradiation dosage. This research is focused on the re-use of N95 masks, but we have provided a source that relates to the reuse of surgical masks using UV-C [25]. There is currently less information regarding the reuse of surgical masks, so if you are interested in implementing this type of device to reuse surgical masks, please understand that there is less information on the interaction of UV-C doses and surgical mask materials. If you are using surgical mask our team recommends you to review the following paper: " Germicidal Ultraviolet Light Does Not Damage or Impede Performance of N95 Masks Upon Multiple Uses.”
Similarly, not much is known about COVID-19, because of this, it is critical that each team reviews and monitors new research articles and FDA guidelines for any change in UV-C dosage and to scientifically support any design changes you make. According to a 2018 study, the research conducted by Mills et al. suggests that dosages past 1J/cm^2 produce diminishing returns for decontamination of N1N1 influenza on N-95 masks [24,25,26]. More recent studies have supported this claim for at least a 3-log reduction of SARS-CoV-2 with at least 1J/cm^2 UV-C irradiation dose [26]. Because of this, 1J/cm^2 was chosen as the ideal irradiation dose for N95 masks, however, this could change as additional research is completed.
A list of helpful sources is available in the ‘Project_Re-Mask_Sources.pdf’ document listed at the end of the page. This is an excellent resource to check if you need more background information on UV-C irradiation, design suggestions, material compatibility, dosimeters, UV-C and N-95 material compatibility, current use of UV-C in hospitals, and other scientific articles used throughout our research.
*Our team can not certify that the 100% of any contaminate on the mask is eliminated due to limitations of the investigation and research.
Similarly, not much is known about COVID-19, because of this, it is critical that each team reviews and monitors new research articles and FDA guidelines for any change in UV-C dosage and to scientifically support any design changes you make. According to a 2018 study, the research conducted by Mills et al. suggests that dosages past 1J/cm^2 produce diminishing returns for decontamination of N1N1 influenza on N-95 masks [24,25,26]. More recent studies have supported this claim for at least a 3-log reduction of SARS-CoV-2 with at least 1J/cm^2 UV-C irradiation dose [26]. Because of this, 1J/cm^2 was chosen as the ideal irradiation dose for N95 masks, however, this could change as additional research is completed.
A list of helpful sources is available in the ‘Project_Re-Mask_Sources.pdf’ document listed at the end of the page. This is an excellent resource to check if you need more background information on UV-C irradiation, design suggestions, material compatibility, dosimeters, UV-C and N-95 material compatibility, current use of UV-C in hospitals, and other scientific articles used throughout our research.
*Our team can not certify that the 100% of any contaminate on the mask is eliminated due to limitations of the investigation and research.
Background:
Ultraviolet Germicidal Irradiation uses UV-C light to damage pathogen’s DNA therefore inactivating it [21]. This process depends on the irradiation dose and the wavelength of the light [26]. Therefore, when a surface receives a UV-C wavelength for a certain irradiation does, any pathogens on the surface are inactivated. However, because direct exposure is necessary for this process, shadowing and angle of impact can have a significant impact on the dosage the surface receives [25,26]. It is critical to prevent and recognize that shadowing from pleating fabric, straps, or writing from markers will lower the UV-C dose received by the material below it [26]. Additionally, the distance away from a UV-C lamp and the angle in which the light impacts the N95’s surface decreases the dosage received as observed in Su et. al.’s paper [26]. Once the surface is irradiated, it is well established that UV-C irradiance is gradually lost as the light is reflected, absorbed and scattered throughout the N95’s layers [26]. Therefore, the minimal irradiation does must be absorbed throughout the mask’s layers not just the exterior surface [26]. This dose was determined to be 1J/cm^2, as of the current research provided. When this dose of UV-C is used against COVID-19 it is understood to have a 3-log reduction in the contaminate.
N95 Decontamination Tool Box Design Expectations and Assumptions:
With respect to this project’s N95 decontamination box, it is expected that a N95 mask is placed directly between two UV-C lamps in order to irradiate both inner and outer surfaces at the same time and ideally increase the light scattered between the mask’s layers. Placing the mask directly between the lamps increases the surface area that is perpendicular to the lamp, which reduces the irradiation lost due to slope, distance, and shadowing. The distances between each lamp and their respective far side of the N95 mask is equal so that the middle of the mask is centered in the box. This distance was chosen in order to make sure that they bottom rim and top of the N95 masks would receive the minimum amount of irradiation needed for decontamination. It is assumed and not tested that the time the mask is irradiated will be enough for both lamps to fully irradiate the surface of a N95 mask and scatter enough UV-C light between a mask’s layers.
Results: |
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10 separate 100mJ/cm^2 dosimeter tests were completed at a temperature fluctuating between 70-72F with a lamp-platform distance of 4in.
It was found that it should take 3 minutes and 54 seconds with a confidence interval of 13 seconds for a 100mJ/cm^2 dosimeter to be fully irradiated. |
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From the experimental derived times above, the time it takes to fully irradiate 1J/cm^2 is calculated by multiplying the equivalent times above in seconds by 10 to convert 100mJ to 1J. Then converting the result into minutes and seconds.
It was found that it should take 39 minutes and 7 seconds with a confidence interval of 2 minutes and 11 seconds for the dosimeter to be fully irradiated. |
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Theoretical Calculations: |
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To calculate the theoretical time (excluding losses and lamp preheating) necessary for the UV-C exposure dosage to develop, the UV intensity (W/cm^2) is needed from the manufacturer. This can usually be found using a part number and product catalog.
Irradiance changes with respect to distance and angle [5,]. The formula in Figure 1 is used to equate irradiance and the distance of an object directly below a UV-C lamp. Using the part number, ZW8S15Y-Z288, and catalog of the UV-C lamp used in this experiment, the UV intensity (28μW/cm2) and distance (1m) was found. The theoretical irradiation in the test stand was found in Figure 2, using the height between the UV-C lamp and the resting place of the dosimeter (4in or .102m). |
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Calculating the theoretical time to fully irradiate the dosimeter:Using the formula in Figure 3, one can find both the theoretical time it takes to irradiate both the 100mJ/cm^2 dosimeter, in Figure 4, and the FDA recommended dose 1J/cm^2 for N95 masks contaminated with influenza, in Figure 5 [24,25].
As seen in Figure 4, the theoretical time needed to fully irradiate the dosimeter is 37.16s. This is under the assumption that there is a constant irradiance from the UVC light. |
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Calculating the theoretical time to fully irradiate N95 Masks:Using the equation in Figure 3, it will take 6 minutes and 11 seconds to fully irradiate a directly perpendicular N95mask 0.102m (4in) away from the lamp with a dose of 1J/cm^2 of UV-C light needed for decontamination in theoretical conditions. This calculation can be seen in Figure 5.
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Experimental Setup:The test stand used in this project functions the same as the decontamination tool box described in the Assembly page, but contains only one UV-C lamp and a camera mounted above it to continuously record the dosimeter's color change. For each test, the dosimeter is placed approximately 4in away from the lamp inside of an enclosure made of foam board to mitigates visible light intruding inside the test stand. In addition a led light was used to help over illuminate the enclosure with saturated light, this helps mitigate the blue hue that is generated by the UV-C lamp when reviewing the video. The photo below depicts the setup used for this project.
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Test Procedure:
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During experimental tests, a 100mJ/cm^2 dosimeter is placed directly below the UV-C lamp at the same distance from the lamp as the farthest side of the N95 mask. The recording camera is turned on, the testing stand is closed, the lamp turned on, and the room temperature checked. After 15 minutes, the lamp was turned off and recording stopped. The dosimeter’s color is checked visually after removal and needs to be visually confirmed and documented within 10 minutes after each test. Therefore, immediately after stopping the recording, the dosimeter was visually confirmed to be fully irradiated.
The analysis of the dosimeter's color was difficult to quantify, especially under difficult lighting, to make sure that a surface was receiving the maximum dosage. Both Figures 7 and 8 below are fully irradiated dosimeters. However, the color changing center does not exactly match the control color on the bottom outer rim and the excess blue light reflected from the dosimeter to the camera provided a challenge when visually inspecting the videos as seen in Figure 7. This was done to find the earliest moment of full irradiation. Past research has used flatbed scanners, iPhone in lightboxes, Spectrocolorimeters, and Color Muse colorimeter [26, 27]. However, in this experiment, our team used foamboard from a dollar store and a old cell phone, to capture video of the dosimeter changing color due to the UV-C light exposure. Since it is difficult to scan the dosimeter while it was transitioning between colors, the recording was visual inspected for the earliest point where the dosimeter stopped changing color. The color changing rate of the dosimeter is influenced by environmental factors such as temperature and humidity [26]. Therefore, all tests were completed indoors to control room temperature, 70-72F, and mitigate the impact of humidity, this quantity was not monitored. Therefore, the color of the fully irradiated dosimeter at the end of the video was compared with frames throughout the video to find the earliest point where the color stopped changing. Because this method is highly based on human perception of color and therefore susceptible to human error, three separate individuals inspected all ten tests and pooled statistics were applied to their responses. *Our team has conducted additional tests in order to improve and mitigate variables, the data presented is our most recent data. Upon request we will provide anyone with all of our raw data from all previous tests. |
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Data Analysis:
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The times for each video were then translated into seconds and pooled statistics were used to find the true mean time and its confidence interval that it takes to irradiate a 100mJ/cm^2 dosimeter as seen in Figures 9 and 10.
In Figure 10, first the number of tests, N, and the number of reviewers, M, was identified. Then the true mean time to fully irradiate a 100mJ/cm^2 dosimeter was found to be 3 minutes and 54 seconds. Next, the standard deviation was found as well as the standard deviation of the mean. The standard deviation of the mean, 6.369 seconds, was multiplied by the corresponding probability from a t-table with 27 degrees of freedom and a 95% confidence interval. The 95% confidence interval of the time needed to fully irradiate a 100mJ/cm^2 dosimeter is 13.1 seconds. To calculate the time needed to fully irradiate 1J/cm^2, the time to irradiate 100mJ/cm^2 is multiplied by 10, as seen in Figure 11. This procedure was recommended to us by the manufacture of the Intellego dosimeters. This is under the assumption that the irradiation intensity of the bulb is constant. Therefore the time it takes to fully irradiate a 0.1J/cm^2 dosimeter is multiplied by 10. This converts 0.1J/cm^2 (or 100mJ/cm^2)to 1J/cm^2 and one can find the time it takes to fully irradiate a dose of 1J/cm^2. The following is the total time to expose a N95 mask to a dosage of 1Jcm^2 is 39 minutes and 7 seconds with a confidence interval of 2 minutes and 11 seconds, as found in Figure 11. |
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Conclusions:
Overall, it was found that it should take 3 minutes and 54 seconds with a confidence interval of 13 seconds to fully irradiate a 100mJ/cm^2 dosimeter with a 8 Watt T5 tube UV-C lamp with a room temperature between 70-72F. To fully irradiate a N95 mask with 1J/cm^2 of UV-C light, it will take 39 minutes and 7 seconds with a confidence interval of 2 minutes and 11 seconds.
The difference between the theoretical and experimental findings could stem from many losses in the system, as it is not ideal. However, it is assumed that the lamp has a warmup time that slowly increases the irradiation dose emitted which then stabilizes and becomes constant. The time of the warmup is included in the initial time to irradiate the 100mJ/cm^2 dosimeter. It is assumed that this extra time carries over into the calculations for the lamp to fully irradiate 1J/cm^2 and will allow for the N95 mask to be irradiated more than the minimum 1J/cm^2 dose for at least a 3-log reduction of SARS-CoV-2 [26] This study was not able to confirm this as only 100mJ/cm^2 dosimeters, which only measure the total irradiation dose of 100mJ/cm^2 not the irradiation rate. Given further study with access to a radiometer, a decrease in total time needed to irradiate a dose of 1J/cm^2 is to be expected as the warmup period would not be included when converting the 100mJ/cm^2 dosimeter time to the time needed for a 1J/cm^2 dosage.
Additionally, room temperature is another factor that could influence the time necessary to irradiate a dosimeter as observed through preliminary data of the time changing when tests are conducted outside, before the test procedure was standardized and test were conducted indoors at 70-72F.
Due to the equipment limitations, this study was limited in the number and length of tests conducted. Additionally, the intensity (rate of irradiation dose) UV-C irradiated onto the surface was not able to be studied during testing as dosimeters are only able to measure cumulative UV-C exposure, dosage, not dose per second produced. Similarly, only 100mJ/cm^2 dosimeters were used to test the decontamination box as 1J/cm^2 dosimeters were not able to be acquired. (The 100mJ/cm^2 dosimeters are the most common and accessible type of dosimeter currently as of: 2/19/2021)
In future studies, access to more dosimeters, especially 1J/cm^2 dosimeters, and a radiometer would allow further investigation to replicate some of the tests conducted in recent research papers like “Quantitative UV-C dose validation with photochromic indicators for informed N95 emergency decontamination” [26]. This would entail preforming tests with a N95 mask covered in 1J/cm^2 dosimeters and compare the time it takes to fully irradiate all the dosimeters covering the mask with the results of this study. A radiometer would allow for more precise evaluation of the warmup time and the assumption that irradiation rate is constant after this time could be investigated (The manufactures of the UV-C lamps used stated that after warm up the output is constant so we are taking their word, due to the fact we are limited in what we can test for at the moment). A recent study published has shown that there is a decrease in irradiation rate over time in an industrial UV-C N95 decontamination chamber [26] (Our team does not have access to equipment to verify these findings independently, that being said different lamps were used in the papers test and our test). However, our teams investigation follows the methods given by the dosimeter manufacturer, Intellego, for using the 100mJ/cm^2 dosimeters to extrapolate irradiation times. Additional tests would also increase experimental data and provide a lower confidence interval value for the time it takes to fully irradiate the 100mJ/cm^2 dosimeter.
Due to the lack of information about intensity this study makes the following assumptions regarding the bulb and its performance
Since this investigation was completed only using 100mJ/cm^2 dosimeters, the only information that was captured was data regarding the total irradiation of 100mJ/cm^2, and the approximate time that it took. With further research using 1J/cm^2 dosimeters and a radiometer, this phenomenon, as described in the paper provide by the representative of N95DECON, could be better understood with respect to this Re-Mask’s N95 decontamination device [26].
After a collaborative meeting with two representatives from N95DECON, it was found that the assumption about the warmup was correct (that it existed we still only have an approximate time). Our team learned a lot from N95DECON about other methods of investigating the UV-C lamps and dosimeters. This information will be used in future tests to refine and learn more about these systems to help provide more access to these technologies to those who are in need. If you want to learn more about N95DECON feel free to check out their website: here.
*Our team cannot certify that the 100% of any contaminate on the mask is eliminated due to limitations of the investigation and research.
The difference between the theoretical and experimental findings could stem from many losses in the system, as it is not ideal. However, it is assumed that the lamp has a warmup time that slowly increases the irradiation dose emitted which then stabilizes and becomes constant. The time of the warmup is included in the initial time to irradiate the 100mJ/cm^2 dosimeter. It is assumed that this extra time carries over into the calculations for the lamp to fully irradiate 1J/cm^2 and will allow for the N95 mask to be irradiated more than the minimum 1J/cm^2 dose for at least a 3-log reduction of SARS-CoV-2 [26] This study was not able to confirm this as only 100mJ/cm^2 dosimeters, which only measure the total irradiation dose of 100mJ/cm^2 not the irradiation rate. Given further study with access to a radiometer, a decrease in total time needed to irradiate a dose of 1J/cm^2 is to be expected as the warmup period would not be included when converting the 100mJ/cm^2 dosimeter time to the time needed for a 1J/cm^2 dosage.
Additionally, room temperature is another factor that could influence the time necessary to irradiate a dosimeter as observed through preliminary data of the time changing when tests are conducted outside, before the test procedure was standardized and test were conducted indoors at 70-72F.
Due to the equipment limitations, this study was limited in the number and length of tests conducted. Additionally, the intensity (rate of irradiation dose) UV-C irradiated onto the surface was not able to be studied during testing as dosimeters are only able to measure cumulative UV-C exposure, dosage, not dose per second produced. Similarly, only 100mJ/cm^2 dosimeters were used to test the decontamination box as 1J/cm^2 dosimeters were not able to be acquired. (The 100mJ/cm^2 dosimeters are the most common and accessible type of dosimeter currently as of: 2/19/2021)
In future studies, access to more dosimeters, especially 1J/cm^2 dosimeters, and a radiometer would allow further investigation to replicate some of the tests conducted in recent research papers like “Quantitative UV-C dose validation with photochromic indicators for informed N95 emergency decontamination” [26]. This would entail preforming tests with a N95 mask covered in 1J/cm^2 dosimeters and compare the time it takes to fully irradiate all the dosimeters covering the mask with the results of this study. A radiometer would allow for more precise evaluation of the warmup time and the assumption that irradiation rate is constant after this time could be investigated (The manufactures of the UV-C lamps used stated that after warm up the output is constant so we are taking their word, due to the fact we are limited in what we can test for at the moment). A recent study published has shown that there is a decrease in irradiation rate over time in an industrial UV-C N95 decontamination chamber [26] (Our team does not have access to equipment to verify these findings independently, that being said different lamps were used in the papers test and our test). However, our teams investigation follows the methods given by the dosimeter manufacturer, Intellego, for using the 100mJ/cm^2 dosimeters to extrapolate irradiation times. Additional tests would also increase experimental data and provide a lower confidence interval value for the time it takes to fully irradiate the 100mJ/cm^2 dosimeter.
Due to the lack of information about intensity this study makes the following assumptions regarding the bulb and its performance
- The information from the manufacturers catalog is correct (ie. after warmup, the output of the lamp was constant).
- There existed some amount of warm up time (It was assumed to be 2 minutes for the lamp used in this study) [10].
- That the masks would be placed in the box in a relatively horizontal manner (ie. there would be a lamp directly above and below each mask).
- The amount of time to full irradiate 1J/cm^2, from 4 inches away from a lamp, will be enough to fully irradiate a N95 mask.
Since this investigation was completed only using 100mJ/cm^2 dosimeters, the only information that was captured was data regarding the total irradiation of 100mJ/cm^2, and the approximate time that it took. With further research using 1J/cm^2 dosimeters and a radiometer, this phenomenon, as described in the paper provide by the representative of N95DECON, could be better understood with respect to this Re-Mask’s N95 decontamination device [26].
After a collaborative meeting with two representatives from N95DECON, it was found that the assumption about the warmup was correct (that it existed we still only have an approximate time). Our team learned a lot from N95DECON about other methods of investigating the UV-C lamps and dosimeters. This information will be used in future tests to refine and learn more about these systems to help provide more access to these technologies to those who are in need. If you want to learn more about N95DECON feel free to check out their website: here.
*Our team cannot certify that the 100% of any contaminate on the mask is eliminated due to limitations of the investigation and research.
Please contact us at Next_Gen_Engineering@Outlook.com for more information or any questions/suggestions.
Documents:
| project_re-mask_sources.pdf |
| project_re-mask_project_proposal.pdf |
Testing:
General Information
Why it was developedThe Field Procedure was created to reduce the project cost and the number of dosimeters needed to calibrate each device. Only three dosimeters should be needed to verify a single device if one follows the field procedure. More information on how the times in the first table were generated is found on the ‘Research’ page.
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How it worksStarting with the average time it takes for one lamp to fully irradiate a 100mJ/cm^2 dosimeter, one can test their device. After checking the results and referring to the decision tree, testers can iterate until they find the amount of time it takes to fully irradiate their dosimeters using each lamp in their device. One can use the formula given at the bottom of the "Field_Procedure.pdf" to calculate the time it takes to fully irradiate an object with 1J/cm^2 of UVC light.
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Field Procedure
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Using the steps provided in this flow chart along with the data provided in the "Field_Procedure.pdf", the device created can be safely evaluated and the required time for the system to produce 1J/cm^2 can be found.
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Unirradiated 100J/cm^2 dosimeter.
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Fully irradiated dosimeter.
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Where to get dosimeters?
We recommend using these dosimeter brands: