Contamination Limits Explained

Why Contamination Limits Matter

Contamination limits exist to protect everyone who handles a package after it leaves your facility — carriers, cargo handlers, recipients, and the general public. Unlike dose rate limits that control radiation exposure, contamination limits control the spread of radioactive material itself. A package could have a perfectly safe dose rate but still have removable contamination on its surface that transfers to hands, vehicles, or other cargo.

I have seen a facility ship a package that met all dose rate requirements but had a contamination smear on the lid from a spill during preparation. The receiving facility's survey caught it, the carrier was notified, and the truck had to be surveyed and potentially decontaminated before it could accept other cargo. That one smear triggered hours of work and multiple incident reports — all preventable with a wipe test before shipping.

Contamination limits are also the criteria that define Surface Contaminated Objects (SCO). Understanding the difference between non-fixed and fixed contamination, and where the limits are, is essential for both package compliance and SCO classification.

Who Needs to Know This

This applies to anyone who:

• Prepares radioactive material packages for transport
• Performs contamination surveys on packages before shipment
• Receives radioactive material shipments and surveys incoming packages
• Classifies objects as SCO based on surface contamination levels
• Surveys and decontaminates transport vehicles
• Manages a radiation protection program that includes transport activities

Non-Fixed vs. Fixed Contamination

Non-Fixed (Removable) Contamination

Non-fixed contamination is radioactive material on a surface that can be removed by normal handling or wiping. This is what you measure with a wipe (smear) test. It's the primary concern for transport because it can transfer to hands, other surfaces, and the environment.

To measure non-fixed contamination, you wipe a 300 cm² area of the package surface with a dry filter paper, cloth, or smear pad, then count the activity on the wipe using a radiation detector. The result is expressed as Bq/cm² (activity divided by the area wiped).

Fixed Contamination

Fixed contamination is radioactive material on a surface that cannot be removed by wiping under normal handling conditions. It's bound to the surface — chemically attached, absorbed into the material, or trapped under a coating. Fixed contamination is measured indirectly: you measure the total surface activity (by direct instrument reading) and subtract the non-fixed portion (measured by wipe test).

Fixed contamination is less of a transport concern because it doesn't transfer to handlers. The main context where fixed contamination limits matter is SCO classification, where the limits for fixed contamination are much higher than for non-fixed.

Package Surface Contamination Limits

The non-fixed contamination on any accessible external surface of a package offered for transport must not exceed (49 CFR 173.443):

These limits are averaged over any 300 cm² area. They apply to all package types — excepted, Type A, Type B, and industrial packages.

The most common measurement mistake I see is confusing the units. Health physics instruments often read in counts per minute (cpm), not disintegrations per minute (dpm). You need to apply your instrument's efficiency factor to convert cpm to dpm before comparing against the limit. A wipe showing 200 cpm on a detector with 10% efficiency is actually 2,000 dpm — which exceeds the 240 dpm/cm² limit for a 300 cm² wipe by a factor of 8.

Vehicle and Conveyance Contamination

Transport vehicles (trucks, aircraft cargo holds, freight containers) must be surveyed after unloading radioactive material. The same non-fixed contamination limits apply: 4 Bq/cm² for β/γ and low-toxicity α, 0.4 Bq/cm² for other α emitters.

If a vehicle exceeds these limits after unloading, it must be decontaminated before it can be returned to service for other cargo. This is the carrier's concern, but it becomes the shipper's problem when the carrier traces the contamination back to your package and sends you the decontamination bill.

In my experience, vehicle contamination issues almost always trace back to packages that weren't surveyed before shipping. A clean package rarely contaminates a vehicle. The problem starts when a shipper skips the pre-shipment wipe test and an undetected smear transfers to the vehicle floor during loading.

SCO Contamination Limits: A Different Scale

The contamination limits for Surface Contaminated Objects (SCO) are different from package surface limits. SCO classification is defined by contamination levels — the object itself is not radioactive, but radioactive material is on its surface.

SCO has both non-fixed and fixed contamination limits, and they are significantly higher than the package surface limits discussed above. The SCO limits define the boundary between SCO-I and SCO-II categories, which in turn determine the required industrial package type.

For inaccessible surfaces, the limits are the same as SCO-I fixed limits for SCO-I, and the non-fixed plus fixed contamination combined must not exceed 8 × 10⁵ Bq/cm² (β/γ) or 8 × 10⁴ Bq/cm² (α) for SCO-II. All values are averaged over 300 cm² (or the actual area if less than 300 cm²).

The key conceptual difference: package surface limits are about keeping the outside of a clean package clean. SCO limits are about classifying an object that is inherently contaminated so it can be transported in the appropriate industrial package.

Empty Package (UN2908) Contamination

Empty packages (UN2908) have their own internal contamination limits. The non-fixed contamination on internal surfaces, averaged over 300 cm², must not exceed:

• β/γ and low-toxicity α: 400 Bq/cm²
• All other α emitters: 40 Bq/cm²

These internal limits are 100 times higher than the external surface limits. This makes sense — the inside of a closed package doesn't contact handlers. But the external surface of an empty package must still meet the standard 4 / 0.4 Bq/cm² limits.

The most common oversight I see with empty package returns is shippers who check the internal contamination but forget to wipe the outside. A package that was stored in a hot lab for months may have picked up external contamination from the work environment even though the interior was properly cleaned.

How to Perform a Wipe Test

The practical steps for verifying package contamination:

1. Select wipe locations — Choose the most likely contamination areas: seams, closures, handles, the bottom (where it sat during loading), and any areas near fill ports or openings
2. Wipe a 300 cm² area — Use a dry filter paper, smear pad, or cloth. Apply moderate pressure across the surface. One wipe per location
3. Count the wipe — Use an appropriate detector (pancake GM for β/γ, ZnS scintillator or gas-flow proportional for α). Record the reading in cpm
4. Convert to dpm — Divide cpm by the detector's counting efficiency for the isotope of concern. A typical pancake GM has about 10–15% efficiency for most beta emitters
5. Calculate Bq/cm² — Divide the dpm result by the area wiped (up to 300 cm²), then convert dpm to Bq (1 Bq = 60 dpm)
6. Compare to limits — 4 Bq/cm² for β/γ and low-tox α, 0.4 Bq/cm² for other α

How RadShip.com Helps

RadShip.com integrates contamination awareness into the shipping workflow:

• RAMcalc — Classifies SCO material based on contamination levels and determines the correct UN number and industrial package type
• Pre-shipment checklists include contamination survey reminders
• Identifies which emitter category (β/γ vs. α) applies to your radionuclide, so you know which limit to check against

Here's the reality: contamination surveys are not glamorous work, and they're easy to skip when you're in a hurry to get a package out the door. But a contamination event from a skipped survey costs far more time and money than the 5 minutes it takes to wipe and count.

Try it free for 7 days.

Common Questions

What is non-fixed (removable) contamination?

Radioactive material on a surface that can be wiped off. It's measured by taking a wipe (smear) of a 300 cm² area and counting the activity on the wipe. The limit is 4 Bq/cm² for β/γ and low-toxicity α, or 0.4 Bq/cm² for other α emitters.

Do I need to survey every package for contamination?

Yes. Before offering a package for transport, you must verify that the non-fixed contamination on accessible external surfaces does not exceed the limits. This is a regulatory requirement, not a recommendation. Wipe testing representative surfaces is the standard method.

What are the contamination limits for transport vehicles?

Same as package surfaces: 4 Bq/cm² for β/γ and low-toxicity α, 0.4 Bq/cm² for other α emitters. Vehicles must be surveyed after unloading and decontaminated if limits are exceeded.

What is the difference between package contamination limits and SCO limits?

They serve different purposes. Package contamination limits ensure the outside of a package is clean enough for safe handling. SCO limits define how contaminated an object can be and still qualify for transport as a surface contaminated object. SCO limits are significantly higher because the contamination is the reason for the shipment, not an unwanted byproduct.

Why is the alpha emitter limit 10 times stricter?

Alpha particles are more hazardous if ingested or inhaled. While alpha radiation can't penetrate skin, alpha-emitting contamination on hands can be transferred to food or inhaled as dust. The stricter limit reflects the higher biological consequence of internal alpha exposure.

Summary: Your Contamination Survey Checklist

Before shipping a radioactive package:

• ☐ Identify the emitter type (β/γ and low-tox α vs. other α) to determine applicable limit
• ☐ Wipe at least the seams, closures, handles, and bottom of the package
• ☐ Count wipes using an appropriate detector with known efficiency
• ☐ Subtract background from each reading
• ☐ Convert cpm to dpm using detector efficiency, then to Bq/cm²
• ☐ Verify result is below 4 Bq/cm² (β/γ) or 0.4 Bq/cm² (α)
• ☐ For empty packages (UN2908): check internal contamination (≤ 400 / 40 Bq/cm²)
• ☐ Document survey results in shipping records
• ☐ If limits exceeded: decontaminate and resurvey before shipping

Regulatory References

DOT Requirements:

• 49 CFR 173.443 — Contamination control (package and vehicle limits)
• 49 CFR 173.403 — Definitions (SCO contamination limits, non-fixed/fixed)
• 49 CFR 173.428 — Empty packaging (UN2908) requirements including internal contamination

Related RadShip Guides:

• UN2912/2913: LSA and SCO
• UN2908/2909: Empty Packages
• Industrial Packages (IP-1, IP-2, IP-3)
• What is Radioactive Material per DOT?