Still Air Box vs Laminar Flow Hood: Choosing and Using a Clean Workspace

Every mushroom grower eventually runs into the same problem: mould. You sterilise a jar of grain or pour an agar plate, open it for a few seconds to add a culture, and a week later it is green. Most of the time, the contamination came from the air around you, from your hands or from the tools you used.

That’s why serious growers do their sterile work in a cleaner space. For home growers there are two main options: a still air box (SAB) and a laminar flow hood. This guide explains how each one works, what it costs, what jobs it suits, and how to use it well. It is a buyer’s and user’s guide in one.

Why a clean workspace matters

Moulds and bacteria are everywhere, and their spores are tiny. Penn State Extension’s guide to green mould shows how much damage one contaminant can do. Trichoderma moulds cause green mould in button mushroom farms, and Penn State reports crop losses of 30 to 100% in Pennsylvania between 1994 and 1996. It also notes that green mould causes significant yield losses in shiitake and oyster mushrooms. On farms, the mould seemed to arrive mainly through contaminated workers and equipment.

A home grower is not running a farm, but the principle is the same. The moments when sterile material is open to the room are when contamination gets in. A clean workspace aims to reduce what can land on your work during those moments. Our contamination guide covers how to recognise the common moulds once they appear.

Option 1: The still air box

What it is

A still air box is a clear, enclosed box with holes for your arms. You work inside it with the lid closed. North Spore, which sells a fold-up version called the NocBox, describes the principle as “limiting air turbulence and providing a clean working environment.” The idea is simple: spores and dust drift on air currents. If the air inside the box is still, fewer of them move around and land on open plates or jars.

What it costs

A still air box is the budget option. North Spore describes it as a simple, inexpensive alternative to a flow hood. Many growers make their own from a large clear plastic storage tote by cutting two arm holes in one long side. The only real requirements are that the tote is clear enough to see through and big enough to hold your work and move your hands.

How to use a still air box

North Spore’s instructions for its NocBox give a good general routine:

1. Clean the inside. Wipe it with 70% isopropyl alcohol and let it evaporate. Make sure the surface is free of debris.
2. Wipe down everything going in. Inspect and wipe jars, bags and plates, and clean instruments with alcohol wipes.
3. Let the air settle. After placing your materials inside, North Spore says to “wait a few minutes… to allow the air to settle in the chamber.”
4. Dress for it. Wear nitrile or latex gloves, and avoid touching your skin, hair or anything unsterilised. North Spore also suggests a lab coat, mask and eye protection.
5. Keep your arms in. “Try to only take your hands out when you are finished.” Each time you pull your arms out and push them back in, you move air.
6. Work quickly with open containers. Keep agar plates and jars open for as short a time as possible.

Flame and alcohol: a safety warning

Alcohol vapour is flammable, and a closed box traps it. North Spore does not recommend flame sterilising inside its NocBox, and warns: “if your NocBox is sealed, alcohol vapors will not be able to leave the NocBox and will cause an explosion risk if any heated element or flame is introduced.” The same caution applies to any homemade box. Let alcohol evaporate fully and let the vapour clear before using any flame or heated tool, or sterilise tools outside the box.

What a still air box is good for

North Spore lists tasks it suits, including:

• Inoculating grain jars and grain bags
• Agar work and liquid culture transfers
• Cloning mushroom tissue
• Making spore prints, as described in our spore print tutorial

Its main limits are space and comfort. You work through holes with restricted movement, and large bags or many plates soon fill the box.

Option 2: The laminar flow hood

What it is

A laminar flow hood takes the opposite approach. Instead of keeping the air still, it blows a steady stream of filtered air across the work surface. GroCycle’s guide to building one explains that the air passes through a HEPA filter and then “moves in parallel lines vertically or horizontally across the work surface”, pushing contaminants away from your work.

The key specifications

Specification	What GroCycle recommends
Filter	H13 or H14 HEPA, which “remove more than 99.97% of particles at 0.3 microns”
Filter depth	At least 3 inches (75 mm) for steadier airflow
Air speed at the filter face	About 100 feet per minute (0.5 metres per second)
Fan size	Filter area multiplied by air speed. For a 2 × 2 ft filter: 4 sq ft × 100 ft/min = 400 cubic feet per minute
Pressure	Total resistance of about 1.0 to 1.2 inches of water with a prefilter, so choose a fan that delivers enough air at that pressure
Working size	At least 2 × 2 ft (61 × 61 cm) for inoculating substrate bags
Filter life	About 3,000 to 5,000 hours of use. Change the prefilter at least yearly

GroCycle also notes that both too much and too little airflow create turbulence, which shrinks the clean area in front of the filter. It suggests horizontal units for bulk substrate bags and vertical cabinets for agar and liquid culture.

Buy or build?

Commercial hoods are not cheap. For example, North Spore sells a 26 × 26 inch mycology flow hood for $650 (price checked in October 2026). Its product page lists a variable speed dial and suggests checking that the airflow is smooth with light smoke or a tissue.

A DIY hood can work, but it isn’t automatically cheaper. GroCycle says that labour and materials for a home build “could easily exceed the price of a pre-made unit”. A DIY build needs a solid box of plywood or MDF, a sealed plenum behind the filter, a suitable fan and a prefilter. GroCycle stresses sealing every joint with silicone so that air only leaves through the filter. An older Make: magazine project by Philip Ross takes a simpler route and builds a small hood from a household air purifier fitted into a sealed plastic box.

How to work in front of a flow hood

• Keep everything clean. Philip Ross’s advice in Make: is to clean the room and all your equipment “so that you could literally eat off of any surface”. He also recommends choosing a work area away from open windows, draughts, plants, pets and other people.
• Don’t touch what you don’t need to. Ross lists “your face, phone, doorknobs” as things to avoid.
• Keep the airflow path clear. Your hands and tools should never be between the filter and the open plate or jar, because anything upstream of your work sheds particles onto it.
• Protect the filter. GroCycle recommends a cover panel over the filter when the hood isn’t in use.

A flow hood protects your work, not you

This point is often missed. The University of Delaware’s environmental health and safety office describes a laminar flow clean bench as a device that blows HEPA-filtered air across the work surface towards the user. It says such benches “protect the product or research from contamination, but they do not protect personnel or the environment.” A flow hood is not a biosafety cabinet, so never use it to contain anything hazardous. And if you work with a lot of spores, remember the air is blowing towards your face.

Still air box vs flow hood at a glance

	Still air box	Laminar flow hood
How it works	Keeps the air still so particles stay put	Blows HEPA-filtered air over the work
Cost	Low. A homemade tote costs little	Often hundreds of pounds or dollars
Space	Small, enclosed and cramped	An open bench, easy to work at
Best for	Occasional agar plates, jars and grain bags	Regular agar work, cloning, many bags at once
Main risks	Moving air with your arms; alcohol vapour trapped near flames	Poor airflow from a bad build; blows spores at the user

Which should you choose?

If you are new to sterile work, start with a still air box. It is cheap, it teaches good habits, and it is enough for many home growers. Our guide to agar and liquid culture describes the kind of work it suits.

Consider a flow hood if you are pouring agar every week, making lots of grain spawn, or tired of working through arm holes. Whichever you use, it only works alongside properly sterilised materials. A clean workspace cannot rescue a jar that wasn’t sterilised in the first place, as our article on sterilisation versus pasteurisation explains.

Good habits for any clean workspace

• Clean the workspace before and after each session.
• Wipe down jars, bags and tools before they go in.
• Work at a calm, steady pace. Fast, jerky movements stir up air.
• Keep open containers covered for as long as possible.
• Label everything with the date and what it is, so you can trace problems.
• Track your results. If one batch keeps failing, look at what changed.

Sources and further reading

• North Spore (2023). How to Use a NocBox Still Air Box for Mushroom Cultivation
• Sayner, A. (2025). How to Build a Laminar Flow Hood for Mushroom Growing: DIY Guide. GroCycle
• North Spore: Biological Concepts Mycology Flow Hood (product page)
• Ross, P. (2012). Home Mycology Lab. Make: magazine
• University of Delaware Environmental Health & Safety: Laminar flow clean benches
• Beyer, D. M., Wuest, P. J. and Anderson, M. G. Green Mold of Mushrooms. Penn State Extension
• CDC Public Health Image Library, image 30484: vertical laminar flow cabinet

Featured image: Trichoderma viride by T. Pedro Hafermann via iNaturalist (CC0 1.0). Inline illustration: CDC/Joseph Barker, CDC Public Health Image Library (public domain).