Docket Number EPA-HQ-OPP-2007-1140, www.regulations.gov

Interim Registration Review Decision for Silica Gel and Silicon Dioxide

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Table of Contents

Contents

I. Introduction ............................................................................................................................... 2

II. Use and Usage........................................................................................................................... 4

III. Scientific Assessments.............................................................................................................. 4

 A. Human Health Assessment ............................................................................................ 4

 Hazard Characterization and Dietary Risk .................................................................... 4

 Residential and Occupational Exposure ........................................................................ 4

 Aggregate Exposure and Risks...................................................................................... 4

 Food Tolerances............................................................................................................. 5

 Human Incidents............................................................................................................ 5

 B. Environmental Assessment ............................................................................................ 5

 Environmental Fate and Exposure................................................................................. 5

 Ecological Risk Conclusion........................................................................................... 5

 Endangered Species Assessment ................................................................................... 6

 Ecological Incidents....................................................................................................... 8

 C. Endocrine Disruptor Screening Program ....................................................................... 8

IV. Interim Registration Review Decision.................................................................................... 9

V. Next Steps and Timeline....................................................................................................... 10

VI. Appendix: Informal Endangered Species Consultation........................................................ 10
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I. Introduction

This document is the Environmental Protection Agency’s (EPA or the Agency) Interim 

Registration Review Decision for silica gel (PC code 072602) and silicon dioxide (PC code 

072605), which are commonly referred to as: the silicates, silica silicates or diatomaceous earth 

(DE), and is being issued pursuant to 40 CFR sections 155.56 and 155.58 (case 4081). A 

registration review decision is the Agency’s determination whether a pesticide meets, or does not 

meet, the standard for registration in the Federal Insecticide, Fungicide, and Rodenticide Act 

(FIFRA). The EPA may issue, when it determines it to be appropriate, an interim registration 

review decision before completing a registration review. Among other things, the Interim 

Registration Review Decision may: 1) require new risk mitigation measures; 2) impose interim 

risk mitigation measures; 3) identify data or information required to complete the review; and 4)

include schedules for submitting the required data, conducting the new risk assessment and 

completing the registration review. Further information on silica gel and silicon dioxide, can be 

found in EPA’s public docket (EPA-HQ-OPP-2007-1140) at www.regulations.gov.

FIFRA, as amended by the Food Quality Protection Act (FQPA) of 1996, mandated the 

continuous review of existing pesticides. All pesticides distributed or sold in the United States 

generally must be registered by the EPA based on scientific data showing that they will not cause 

unreasonable risks to human health or to the environment when used as directed on product 

labeling. In 2006, the Agency implemented the registration review program pursuant to FIFRA 

section 3(g) and will review each registered pesticide every 15 years to determine whether it 

continues to meet the FIFRA standard for registration. The registration review program is 

intended to make sure that, as the ability to assess and reduce risk evolves and as policies and 

practices change, all registered pesticides continue to meet the statutory standard of no 

unreasonable adverse effects. Changes in science, public policy, and pesticide use practices will 

occur over time. Through the registration review program, the EPA periodically re-evaluates

pesticides to make sure that as these changes occur, products in the marketplace can continue to 

be used safely. Information on this program is provided at

www.epa.gov/oppsrrd1/registration_review/.

Silica gel and silicon dioxide were first registered as pesticides in 1956 and 1960,

respectively. The silicates are registered for use as insecticides to control non soil-dwelling 

terrestrial invertebrates through desiccation. They are formulated as dust or wettable powders 

(and sometimes combined with pyrethrins and piperonyl butoxide (PBO)) and have both indoor 

and outdoor registered uses in both commercial (e.g., rights of ways, nursery greenhouses and 

industrial sites) and residential settings (e.g., landscape maintenance, lawn and structural pest 

control).

Pursuant to 40 CFR §155.50, the EPA initiated registration review for silica gel and 

silicon dioxide in March 2008, when EPA opened the silica gel and silicon dioxide registration 

review public docket (docket number EPA-HQ-OPP-2007-1140, accessible at

www.regulations.gov.) The following timeline highlights significant events that have occurred 

during the registration review of silica gel and silicon dioxide and can be found in the public 

docket (EPA-HQ-OPP-2007-1140), accessible at www.regulations.gov.
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• March 26, 2008 – The EPA published the Silica and Silicates Summary Document, dated 

March 14, 2008 for a 60-day public comment period. The Summary Document included 

the EPA’s Preliminary Work Plan for Silica and Silicates, the Problem Formulation for 

Ecological Risk Assessment for Silicon Dioxide and Silica Gel, dated November 16, 2007

and the HED Registration Review Scoping Document, dated November 16, 2007. 

• August 5, 2008 – The EPA published the Silica and Silicates Final Work Plan. The EPA

indicated that it would conduct a comprehensive ecological risk assessment that included 

an endangered species risk assessment, and that EPA would not conduct a human health 

risk assessment for the silica silicates registration review. 

• December 10, 2009 – The EPA issued the Generic Data Call-In Notice (GDCI-072602-

847) for silica gel and silicon dioxide. Data required by the EPA included:

o GLN 830.7050: UV/Visible absorption

o GLN 850.2100: Avian acute oral toxicity test

• March 7, 2013 – The EPA published the Environmental Fate and Ecological Risk 

Assessment for the Registration Review of Silica, dated January 16, 2013 for a 60-day 

public comment period, during which time, no comments were received.

• June 30, 2015 – The EPA published the Silica Gel and Silicon Dioxide Proposed Interim 

Registration Review Decision, dated June 24, 2015, for a 60-day public comment period, 

during which time, no comments were received.

• December 2015 - The Agency is now publishing the Silica Gel and Silicon Dioxide 

Interim Registration Review Decision accessible on www.regulations.gov under docket 

number EPA-HQ-OPP-2007-1140.

II. Use and Usage

Silica gel and silicon dioxide are both derived from diatomaceous earth (DE) deposits 

which are commonly found in most soils. Silicon dioxide is diatomaceous earth, formed from a 

naturally-occurring material composed of the shells of minute, single-celled algae. Unlike 

silicon dioxide, silica gel has no registered outdoor uses. Silica gel is characterized as the 

amorphous form of silicon dioxide, which is made by the gelation of sodium silicate and sulfuric 

acid. Because silica gel has no outdoor uses, the EPA considered environmental exposure solely 

from silicon dioxide when conducting its environmental assessment.
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III. Scientific Assessments

A. Human Health Assessment

A summary of the EPA’s human health analysis is presented below. For detailed 

information on EPA’s human health assessment for silica gel and silicon dioxide, see the Silica 

and Silicates Registration Review Summary Document, dated March 14, 2008, available in the 

public docket found at EPA-HQ-OPP-2007-1140-0005, which can be accessed at 

http://www.regulations.gov/. 

Hazard Characterization and Dietary Risk

No toxicological endpoints were identified in the silica gel and silicon dioxide hazard 

database due to its low toxicity. The EPA believes that exposure to silica gel and silicon dioxide 

from direct or indirect application to food commodities, or from residues in food handling and 

food preparation areas, would not result in risks of concern. The Food and Drug Administration 

(FDA) has classified silicon dioxide and silica gel as Generally Recognized as Safe (GRAS)

when used as food additives (21 CFR 182.90 and 182.1711, respectively). In addition, silicon 

dioxide has been exempted from the requirement of a tolerance when applied to: growing crops,

raw agricultural commodities after harvest, and animals (40 CFR 180.1017 (a) and (b)); and in 

food and feed processing and storage areas (40 CFR 185.1700 and 186.1700). Based upon these 

considerations, EPA determined that a quantitative human health risk assessment was not needed 

for silica gel and silicon dioxide, and therefore a dietary (food + drinking water) assessment was 

not conducted. 

Residential and Occupational Exposure

Silica gel and silicon dioxide are registered for homeowner uses (including crack and 

crevice treatment, pets, and etc.); therefore, there is the potential for applicator and postapplication residential exposures to occur. However based on the low toxicity of the silica gel 

and silicon dioxide, the EPA has determined that residential exposures to these compounds 

would not be of concern, and, consequently, neither a residential handler nor a residential postapplication assessment are needed.

While silica gel and silicon dioxide labels require the use of a respirator to protect 

against irritating particles, the EPA has not identified any toxicological endpoints appropriate for 

a quantitative occupational assessment. Therefore, the EPA believes that neither occupational 

handler nor occupational post-application exposures would be of concern, and has concluded that 

an occupational assessment is not needed. 

Aggregate Exposure and Risks

Due to the EPA’s determination not to conduct either a dietary (food + drinking water) or 

residential risk assessment for the silica silicates, an aggregate risk assessment was not needed.
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Food Tolerances

Silica gel and silicon dioxide have been exempted from the requirement of a tolerance 

when applied to growing crops and raw agricultural commodities after harvest (40 CFR 

180.1001 (c) and (d), to growing crops, raw agricultural commodities after harvest and animals 

(40 CFR 180.1017); and in food and feed processing and storage areas (40 CFR 185.1700 and 

186.1700).

Human Incidents

Based on a search conducted on June 2, 2015, the Agency’s Incident Data System (IDS) 

has approximately 280 reported incidents associated with silica gel and silicon dioxide (between 

January 1, 2010 and June 2, 2015). Of the reports identified in IDS data base, 11 cases involved 

either silica gel or silicon dioxide (10 of moderate severity, and 1 of major severity). The one 

major severity case involved an 18-year old female who started experiencing migraines and 

seizures about a week after her mother treated their home (floorboards and baseboards 

throughout).

On June 2, 2015, the Agency also consulted the Centers for Disease Control and 

Prevention/National Institute for Occupational Safety and Health (CDC/NIOSH) Sentinel Event 

Notification System for Occupational Risk (SENSOR) database, and identified 138 cases from 

1998 to 2011 involving silica gel and silicon dioxide. Of the reports identified in the SENSOR 

database, 9 cases involved a single active ingredient, either silica gel or silicon dioxide (all cases 

were of low severity). Also, all 9 cases were worker related, 5 of which were related to a single 

work event where silica dust was applied, and subsequently vacuumed in an office area while 

employees were present. 

B. Environmental Assessment

A summary of the EPA’s environmental assessment for silica gel and silicon dioxide is 

presented below. For detailed discussions of all aspects of the environmental risk assessment, 

see the Environmental Fate and Ecological Effects Risk Assessment for the Registration Review 

of Silica, (dated January 16, 2013), available in the public docket EPA-HQ-OPP-2007-1140

which can be accessed on http://www.regulations.gov/. 

Environmental Fate and Exposure

While silica gel and silicon dioxide are registered for use on a variety of agricultural 

crops, turf, industrial sites and homeowner uses, it is rarely broadcast applied, but more often 

applied as a spot treatment. Therefore, movement of silica gel and silicon dioxide away from the 

treatment site via wind or rain is expected to be limited. Further, USGS (1984) reports that the 

silica silicates are components of U.S. soils at approximately 2–31%. While silica gel and 

silicon dioxide can potentially be transported to aquatic environments, exposure to aquatic 
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organisms is not considered to be a risk since the mode of action of the silica silicates is negated 

when wet (i.e., upon becoming wet, silica gel and silicon dioxide will clump, and thereby lose 

the ability to cut and desiccate target organisms). 

Ecological Risk Conclusions

1. Terrestrial Species

Based upon the low toxicity of the silica silicates, its ubiquitous presence in the 

environment, and its limited use as a pesticide, the EPA determined that a quantitative ecological 

assessment was not required for registration review.

Based on available data, terrestrial vertebrates (including birds) are unlikely to be harmed 

from either dietary or dermal exposure to these compounds. Further, because of the ubiquitous 

nature of the silica gel and silicon dioxide, terrestrial invertebrates living on and in the soil are 

likely to have adapted to the these compounds. However, EPA does expect that terrestrial 

invertebrates that do not live in or on the soil, (e.g., flying or leaf-dwelling insects), may be 

adversely affected by silica gel and silicon dioxide due to its insecticidal properties (i.e., to cut 

and desiccate). 

2. Aquatic Organisms

As noted above, because the mode of action is negated when silica gel and silicon dioxide 

become wet, the EPA does not anticipate that these compounds will adversely affect aquatic 

organisms (fish or invertebrates). 

3. Plants

Because terrestrial plants are likely to be routinely exposed to silica, as windborne 

particles, and because both terrestrial and aquatic plants produce a form of silica (biogenic 

amorphous silica), adverse effects to plants (either terrestrial or aquatic) are not anticipated from 

exposure to silica gel or silicon dioxide.

Endangered Species Assessment

The ecological risk assessment supporting the EPA’s registration review interim decision 

for silica gel and silicon dioxide (as diatomaceous earth) (DE) contains a completed Endangered 

Species Act (ESA) analysis, including effects determinations for endangered and threatened 

(listed) species and their designated critical habitat, and incorporates concurrence from the U.S. 

Fish and Wildlife Service (FWS) on EPA’s “Not Likely to Adversely Affect (NLAA)”

determination, in informal consultation. 

While EPA did not conduct a quantitative analysis of silica gel and silicon dioxide for 

either listed or non-listed species as part of its ecological assessment, EPA made a “No Effect” 

determination for all listed species, with the exception of 57 listed terrestrial invertebrate species 
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for which an NLAA finding was determined. The EPA reached this conclusion based upon the 

following rationale: 

1. Silica gel and silicon dioxide have a mechanical rather than a toxic mode of 

action; therefore, the potential for adverse effect is limited in time and space, 

to the treated area, not off-site, and to a short interval after applications, since 

weathering renders silica gel and silicon dioxide inefficacious. Although the 

time to effective weathering is not quantified, it is expected to be no longer 

than to the next rainfall.

2. Silica gel and silicon dioxide are naturally occurring substances, present in 

many soils at low concentrations. Thus, listed species are expected to be 

exposed to these compounds and are presumably adapted to low exposure 

concentrations.

3. It is uncertain whether listed terrestrial invertebrate species would visit treated 

areas, or remain on these areas during the time of application, due to the 

attendant human disturbance.

4. Silica gel and silicon dioxide are used in organic agriculture and by 

homeowners seeking less-toxic alternatives to conventional pesticides. To the 

degree that silica gel and silicon dioxide are used in place of conventional 

insecticides, EPA believes that the silicates are a much safer alternative in 

terms of any potential impacts to listed terrestrial invertebrates.

On February 10, 2014, pursuant to section 7 of the ESA, EPA requested informal 

consultation with the FWS and concurrence that silicon dioxide and silica gel is NLAA 57 

identified listed terrestrial invertebrates.

On April 2, 2014, the FWS issued a letter to EPA indicating its concurrence with the 

EPA’s NLAA determinations for silica gel and silicon dioxide on 57 listed terrestrial 

invertebrates. In an email that preceded the April 2, 2014 letter, the FWS noted that one species, 

the Ash Meadows naucorid (Ambrysus amargosus) should be included among the listed species 

identified, and that there is “No Effect” to this species from silica gel and silicon dioxide ( email 

communication from Patrice Ashfield to Anita Pease, March 31, 2014).

Subsequent to the issuance of FWS’s concurrence letter, the FWS Oregon Field Office 

highlighted potential concerns for specific silica gel and silicon dioxide use sites (e.g., pastures, 

vineyards) for two listed species of butterfly, namely the Fender’s blue butterfly (Icaricia 

icarioides fenderi) and the Taylor’s checkerspot butterfly (Euphydryas editha taylori). This 

concern resulted in further work between EPA and FWS to ensure no adverse effects to these 

two species of butterfly from exposure to DE use in pastures and/or vineyards. EPA provided 

additional information to the FWS regarding existing state-level pollinator protection 

requirements for silica gel and silicon dioxide, which are triggered by a state decision to register 

silica gel and silicon dioxide for a new use (via a section 18 or 24c label), and similar guidance is 

available through other extension services. EPA also provided additional data showing low 

toxicity of silica gel and silicon dioxide to bees (as a surrogate for butterflies) at rates exceeding 

current application levels. Based on this additional information provided by EPA, the FWS 
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reaffirmed their concurrence with EPA’s NLAA determination for the Fender’s Blue and 

Taylor’s checkerspot butterflies (see Appendix for letter from Craig Aubrey of U.S. FWS to 

Anita Pease of U.S. EPA dated March 9, 2015).

EPA does not anticipate conducting any further analysis of silica gel and silicon dioxide 

for potential risks to listed species and/or designated critical habitat unless additional data or 

information are obtained that would otherwise inform this determination.

Ecological Incidents

As of May 16, 2012, EPA searched its ecological incident databases, including the 

Ecological Incident Information System (EIIS), the Avian Incident Monitoring System (AIMS), 

and the Aggregate Incident Reports database. In the EIIS, one ecological incident was identified 

that occurred in Texas in 1994 and involved six fish with possible exposure to silica gel. The 

incident was reported as an “intentional misuse” and considered “possibly” caused by silica gel,

since there are no outdoor uses of silica gel. Furthermore, the product associated with the 

incident contained a pyrethroid and PBO. No ecological incidents were reported through the 

AIMS or the Aggregate Incident Reports database.

 C. Endocrine Disruptor Screening Program

As required by FIFRA and the Federal Food, Drug, and Cosmetic Act (FFDCA), EPA 

reviews numerous studies to assess potential adverse outcomes from exposure to chemicals. 

Collectively, these studies include acute, subchronic and chronic toxicity, including assessments 

of carcinogenicity, neurotoxicity, developmental, reproductive, and general or systemic toxicity. 

These studies include endpoints which may be susceptible to endocrine influence, including 

effects on endocrine target organ histopathology, organ weights, estrus cyclicity, sexual 

maturation, fertility, pregnancy rates, reproductive loss, and sex ratios in offspring. For 

ecological hazard assessments, EPA evaluates acute tests and chronic studies that assess growth, 

developmental and reproductive effects in different taxonomic groups. As part of its most recent 

registration decision for the silicates, EPA reviewed these data and selected the most sensitive 

endpoints for relevant risk assessment scenarios from the existing hazard database. However, as 

required by FFDCA section 408(p), silica gel and silicon dioxide are subject to the endocrine 

screening part of the Endocrine Disruptor Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including 

pesticide active and other ingredients) may have an effect in humans or wildlife similar to an 

effect produced by a “naturally occurring estrogen, or other such endocrine effects as the 

Administrator may designate.” The EDSP employs a two-tiered approach to making the 

statutorily required determinations. Tier 1 consists of a battery of 11 screening assays to identify 

the potential of a chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or 

T) hormonal systems. Chemicals that go through Tier 1 screening and are found to have the 

potential to interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP 

where EPA will determine which, if any, of the Tier 2 tests are necessary based on the available 
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data. Tier 2 testing is designed to identify any adverse endocrine-related effects caused by the 

substance, and establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the EPA must screen all pesticide chemicals. Between 

October 2009, and February 2010, EPA issued test orders/data call-ins for the first group of 67 

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list 

of chemicals identified for EDSP screening was published on June 14, 20131 and includes some 

pesticides scheduled for registration review and chemicals found in water. Neither silicon 

dioxide nor silica gel are on either list 1 or list 2. For further information on the status of the 

EDSP, the policies and procedures, the lists of chemicals, future lists, the test guidelines and the 

Tier 1 screening battery, please visit our website.2 

In the interim, EPA is making no human health or environmental safety findings 

associated with the EDSP screening of the silica silicates. Before completing this registration 

review, the EPA will make an EDSP FFDCA section 408(p) determination.

IV. Interim Registration Review Decision

EPA has completed qualitative human health and ecological risk assessments, including 

an endangered species assessment, for silica gel and silica dioxide. The EPA has also updated its 

silica gel and silicon dioxide database by receiving the following data required under the 

registration review GDCI (GDCI-072602-847).

• GLN 830.7050: UV/Visible absorption

• GLN 850.2100: Avian acute oral toxicity

The EPA has determined that no human health risks are expected from the use of 

pesticide products that contain silica gel and silicon dioxide. The EPA has also determined that 

potential adverse ecological effects from the use of pesticide products that contain silica gel and 

silicon dioxide would be expected for terrestrial invertebrates that do not live in or on the soil, 

(e.g., flying or leaf-dwelling insects). The EPA believes that potential effects to these species 

would be limited in time, since the effects of silica gel and silicon dioxide only last until a 

rainfall event occurs (when the rain would cause the compounds to clump and lose its mode of 

action). In addition, EPA believes the potential effects to terrestrial invertebrates would also not 

be geographically widespread since silica gel and silicon dioxide are typically applied as a spot 

treatment. 

In accordance with 40 CFR Sections 155.56 and 155.58, the EPA is issuing this interim 

registration review decision. Pending the outcome of the EDSP analysis for silica gel and silicon 

dioxide, the EPA has made the following interim decision:

 1 See docket number EPA-HQ-OPPT-2009-2009-0477 at http://www.regulations.gov/ for the final second list of 

chemicals.

2 http://www.epa.gov/endo/
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• Because of low toxicity, the EPA conducted qualitative human health and ecological 

analyses and determined that no risks of concern are likely to result from the current 

registrations of either silicon dioxide or silica gel; and no data or label changes are 

needed for either silicon dioxide or silica gel.

• EPA has made a “No Effect” determination under the ESA for all listed species, 

excluding 57 listed terrestrial invertebrate species, for which a NLAA determination 

was made, and no modification of designated critical habitat for these/all species. In 

letters dated April 2, 2014 and March 8, 2015, the FWS indicated its concurrence 

with the EPA’s conclusion that silica gel and silicon dioxide are NLAA the 57 listed 

terrestrial invertebrate species.

V. Next Steps and Timeline

Interim Registration Review Decision

A Federal Register Notice will announce the availability of this interim registration 

review decision for silica gel and silicon dioxide. A final decision on the silica gel and silicon 

dioxide registration review case will occur after an EDSP FFDCA section 408(p) determination.

VI. Appendix: Informal Endangered Species Consultation

A. Review of Informal Consultation on the Proposed Registration of Silicon Dioxide 

and Silica Gel for Two Butterfly Species (letter from FWS dated March 9, 2015)

B. Informal Consultation on the Proposed Registration of Silicon Dioxide and Silica 

Gel (letter from FWS dated April 2, 2014)