- What the Credential Actually Is
- Who Issues It and Why That Matters
- The Six Exam Domains
- Exam Format and Test-Day Logistics
- Eligibility Pathways and the Application File
- Fees, Term and Renewal
- Reading the Pass Rate Correctly
- The Reference Library Behind the Exam
- Where the Credential Fits in Practice
- Sequencing Your Preparation by Domain
- Frequently Asked Questions
- CMI here means Council-certified Microbial Investigator, issued by the American Council for Accredited Certification (ACAC) and accredited by CESB.
- The exam is 100 multiple-choice questions in three hours, closed book, with a 75% passing score.
- Initial cost is $500 for a two-year term: $100 exam plus $400 certification fee.
- Six official domains apply, but ACAC publishes no percentage weights for them.
What the Credential Actually Is
The Council-certified Microbial Investigator credential certifies that a professional can design, conduct and interpret microbial sampling regimens in the built environment. It is a performance-oriented, experience-gated certification rather than a course completion certificate. You do not earn it by sitting through a class; you earn it by documenting real field work, passing a proctored written examination and winning approval from a certification board.
Because the acronym is shared by several unrelated credentials in other industries, it pays to be precise. This article covers only the ACAC credential for microbial consulting and investigation. If you have seen "CMI" attached to a different certifying body, a different exam or different fees, that is a separate credential and none of its details apply here. For related plain-language explainers, see our pages on what CMI stands for and the meaning of CMI.
Who Issues It and Why That Matters
The certification is administered by the CMI Certification Board of the American Council for Accredited Certification. The program holds accreditation from the Council for Engineering and Scientific Specialty Boards (CESB), which is a meaningful signal for clients, employers and regulators who want a third-party-accredited credential rather than a vendor badge.
ACAC publishes a candidate handbook, an exam study guide, a formal domain chart, fee schedules, application instructions and recertification procedures. Those documents are the authoritative sources for this credential. Where they disagree with each other, the formal approved domain chart controls the exam content outline, and later sections of this article flag the places where ACAC's own pages differ.
The Six Exam Domains
The current formal chart, approved February 21, 2024, lists six exam domains. Each contains numbered subdomains (columns 1 through 7 on the chart), but those columns are subdomains and not additional domains. ACAC does not publish percentage weights for the domains, so any claim that one domain is "highest-weighted" is not supported by the issuer. For a deeper walkthrough, read our complete guide to all 6 CMI content areas.
Domain 1: Scientific knowledge of biological agents
The biology foundation: what the organisms are, how they grow and what conditions sustain them.
- Fungi, bacteria and other biological contaminants relevant to indoor environments
- Moisture, nutrients and temperature as growth drivers
- How biological material becomes airborne or settles on surfaces
Domain 2: Potential health effects and risks associated with exposure
Connecting exposure to human outcomes without overstating certainty.
- Allergic, irritant and infectious pathways of exposure
- Susceptible populations and how risk differs across occupants
- The limits of what sampling data can say about health
Domain 3: Principles of Microbial Investigation
The reasoning layer: how an investigator frames a problem before collecting anything.
- Building and moisture-history assessment
- Forming hypotheses and choosing what to test
- Documenting findings and communicating conclusions
Domain 4: Principles of Microbial Sampling
The core of the credential, matching the experience requirement of designing, conducting and interpreting sampling regimens.
- Selecting air, surface and bulk sampling approaches for a stated objective
- Sampling design, controls and comparison locations
- Interpreting results against context rather than a single number
Domain 5: Principles of Microbial Remediation
How investigation findings connect to cleanup scope and verification.
- Water damage response and containment concepts
- Remediation protocols and post-remediation evaluation
- Industry standards for water damage restoration and mold remediation
Domain 6: Health & Safety
Protecting the investigator and others on site.
- Respiratory protection programs and respirator selection
- Hazard recognition during inspections and sampling
- Regulatory expectations for personal protective practices
Exam Format and Test-Day Logistics
The examination is delivered through Kryterion, Inc. and is a computer-delivered written exam. The format facts published by ACAC are:
- Length: 100 multiple-choice questions, each with at least four answer options
- Time: three hours
- Rules: closed book
- Passing score: 75%, established using the Modified Angoff method
- Practical component: none described; the exam is written only
The handbook describes a fixed question set with randomized question and answer order, immediate electronic scoring and an emailed result. Adaptive testing is not described. ACAC does not publish how many of the 100 items are scored versus unscored, and calculator permission is not verified, so confirm that point directly with ACAC before test day. For a closer look at how the cut score works, see our page on the CMI passing score.
Delivery options and check-in
Candidates may test at a proctored testing center or online with video proctoring. Testing-center candidates need two government-issued IDs, including one with a photo. Online candidates supply their own computer, an approved webcam and a private location, with at least 2 Mbps upload and download speed; phones and tablets are not permitted for the exam.
One small conflict to plan around: ACAC's examinations page says to arrive 15 minutes early, while the handbook says 30. Arrive at the earlier, longer lead time and the discrepancy never affects you. Scheduling windows are covered in our guide to CMI exam dates and deadlines.
Eligibility Pathways and the Application File
The exam is only one gate. ACAC requires an application showing that you have actually done microbial sampling work. There are two pathways:
| Pathway | Education | Experience |
|---|---|---|
| Pathway A | Related two-year postsecondary degree, or an equivalent 15 qualifying college credits | At least one year of documented microbial sampling |
| Pathway B | High school diploma or GED | Two years of consulting experience in microbial sampling |
Related fields include microbiology, engineering, science, architecture and industrial hygiene. ACAC's eligibility FAQ adds an important clarification: college credits do not replace field time, and at least one full year of actual field experience is required regardless of how many credits you hold. No mandatory preparation course or minimum training-hour count is published, so a course is optional rather than a prerequisite.
What the application includes
- Online application forms with required email verifications
- Four project sheets documenting your sampling work
- Applicable academic evidence
- Experience verification
Candidates must demonstrate that they have designed, conducted and interpreted microbial sampling regimens, so choose projects that show all three rather than only the fieldwork. Approval requires unanimous board approval. For a full eligibility walkthrough, see CMI requirements and how to qualify.
Also note that passing an examination solely for state licensing purposes does not by itself award CMI certification. Certification still requires the full application and board approval.
Fees, Term and Renewal
ACAC's fee schedule, in US dollars, is simple compared with many credentials:
| Item | Fee |
|---|---|
| Examination | $100 |
| Certification (includes a $100 nonrefundable processing fee) | $400 |
| Initial total for the two-year term | $500 |
| Retake | $100 |
| Renewal | $400 |
The $100 processing fee is part of the $400 certification fee, not an additional charge. ACAC does not publish a separate member and nonmember fee schedule. Budget separately for study materials, any optional training and your own time documenting project sheets. Our CMI certification cost breakdown goes into the full picture.
Staying certified
Certification lasts two years. ACAC's recertification procedures call for 40 recertification credits (RCs) during the term: 10 for continued field activity and 30 for professional development. You attest to your credits and retain documentation in case of audit, and a current photo is requested. ACAC's pages are not perfectly aligned here either: the handbook uses general 40-RC wording and the program page mentions 20 professional-development hours per year. Follow the detailed recertification procedures page and keep records from day one rather than reconstructing them at renewal.
Reading the Pass Rate Correctly
ACAC's 2025 Exam Pass Ratio reports an 88% pass rate for this credential, based on 22 passes from 25 examinations. That is a real issuer-published figure, but it deserves careful reading:
- The sample is small: 25 sittings.
- The report includes retakes and state-licensing-only examinations, and ACAC does not disclose how many CMI sittings fall into either category.
- It therefore does not establish first-attempt success and cannot predict your individual result.
- No 2026 pass rate has been verified.
Do not confuse the 88% observed pass rate with the 75% passing score. One describes how a group of candidates performed; the other is the score you must reach on your own exam. ACAC's activity report as of December 31, 2025 also lists 25 CMI applicants, 59 recertifications and 166 current certifications, which gives a sense of how specialized this credential community is. For more, see our analysis of the CMI pass rate data and our difficulty guide.
The Reference Library Behind the Exam
The CMI handbook lists specific references, and some are published with explicit edition alternatives:
- ACGIH, Bioaerosols: Assessment and Control, first edition (1999) or second edition (2024)
- AIHA, Field Guide for the Determination of Biological Contaminants, second edition (2005)
- ANSI/IICRC S500-2021, Standard for Professional Water Damage Restoration, fifth edition
- ANSI/IICRC S520-2015 and IICRC R520-2015 (third editions), or ANSI/IICRC S520-2024, fourth edition, for professional mold remediation
- OSHA, Small Entity Compliance Guide for the Respiratory Protection Standard (2011)
- NIOSH, Respirator Selection Logic (2004)
These listings are bibliographic; they do not tell you which exam revision is in force. The handbook's study guide includes selected pages from the texts, but ACAC warns that those pages are illustrative and that candidates need comprehensive knowledge of the references. The study-guide passages are not exhaustive exam coverage. Also note that the detailed CMI study-guide section does not provide an OSHA section or page subset, so read the respiratory protection material on its own merits for Domain 6.
Key Takeaway
Map each reference to a domain before you read it. The standards for water damage and mold remediation serve Domain 5, the respiratory protection documents serve Domain 6, and the bioaerosol and biological contaminant guides support Domains 1 and 4. Reading a reference without knowing which domain it feeds is how candidates burn hours on low-yield pages.
Where the Credential Fits in Practice
The CMI is aimed at practitioners who carry out microbial assessments: industrial hygiene and environmental consulting staff, building science and engineering professionals who investigate moisture and contamination problems, and consultants working alongside restoration contractors. Because the required experience is specifically about sampling regimens, the credential signals investigative competence rather than remediation labor. Our overview of CMI jobs covers the employer side in more detail.
A related question is how this credential differs from ACAC's companion Microbial Consultant credential. They share an examination chart document, but their eligibility pathways differ, and the advanced education and experience routes that apply to the other credential do not transfer here. If you are comparing the two, read the requirements for each directly rather than assuming they match.
On earnings, no reliable salary survey specific to ACAC CMI holders has been verified, and no causal earnings premium from holding the credential has been established. Compensation depends on your underlying occupation, region and employer. Treat the credential as evidence of competence that supports hiring and client trust, and weigh it against its modest fees in our discussion of whether the CMI is worth it and our salary analysis.
Sequencing Your Preparation by Domain
Method matters less than order here. Since Domain 4 mirrors the experience you already had to document, many candidates find it comfortable, while Domain 6 is easy to neglect because it is smaller in the handbook summary. One reasonable sequence for a four-week plan:
Foundations: Domains 1 and 2
- Review biological agents and exposure pathways from the bioaerosol and field guide references
- Write your own one-page summary of exposure-versus-health limits
Investigation and sampling: Domains 3 and 4
- Rework your own project sheets as exam-style reasoning problems
- Practice choosing a sampling approach for a stated objective and justifying it
Remediation and safety: Domains 5 and 6
- Read the S500 and S520 material against the remediation questions you expect
- Work through the OSHA and NIOSH respiratory documents
Integration and timing
- Take full 100-question sets under a three-hour limit
- Review every miss by domain and revisit the weakest one
Because ACAC publishes no domain weights, spread your time across all six rather than betting on a presumed emphasis. Some third-party sites publish allocation percentages, but those describe their own practice materials, not official exam weights. When you are ready to test yourself, use the CMI practice tests and compare your results against the full study guide and the one-page CMI cheat sheet. You can also revisit the broader CMI certification overview and training options as you plan.
Frequently Asked Questions
It stands for Council-certified Microbial Investigator, a certification issued by the American Council for Accredited Certification (ACAC) through its CMI Certification Board and accredited by CESB. It is unrelated to other credentials that happen to share the acronym.
The exam has 100 multiple-choice questions with at least four options each, delivered in three hours, closed book. The passing score is 75%, set using the Modified Angoff method. The scored versus unscored split is not published.
The initial cost is $500 for the two-year term: a $100 examination fee plus a $400 certification fee that includes a $100 nonrefundable processing fee. A retake is $100 and renewal is $400.
Yes. You need either a related two-year degree or 15 qualifying college credits plus at least one year of documented microbial sampling, or a high school diploma or GED plus two years of consulting experience in microbial sampling. You also submit four project sheets and experience verification.
Certification lasts two years. Renewal involves 40 recertification credits, with 10 for continued field activity and 30 for professional development, plus attestation and retained documentation in case of audit. ACAC's own pages vary slightly in wording, so follow the detailed recertification procedures.