Project FAQs

The team at Planetary is grateful for the engaged participation from communities around the world regarding our work. The following Frequently Asked Questions (FAQs) reflect the themes of the most often raised questions or comments that have arisen from our outreach sessions. Our goal for each question is to briefly address our general approach to projects. We recognise that there are many perspectives at play in our communities and aim to target our responses at the “curious and concerned citizen” with these FAQs. Recognising some groups of people  (e.g. activist, scientist, etc.) may be interested in a more detailed answer, or wish to explore a different facet of the topic, we invite you to reach out to us with your additional questions.

Are these projects safe?

Lead Responder: Mike Kelland
Chief Executive Officer, Co-Founder & Board Member

We, along with our third party scientific and academic partners, are confident that these projects are safe for humans, animals and the marine environment from plankton to whales and everything in between. 

The scale of our research is always carefully calculated to be well within all safety thresholds. Magnesium hydroxide is used in wastewater treatment and is regularly consumed by humans and animals in food and medicines. 

In seawater, magnesium hydroxide reacts with CO2 to form carbonates and bicarbonates – the same stuff that’s in a Rennie or Tums. Magnesium and carbonates are vital and abundant components of seawater. Carbonates are the building materials for shells and bones. Without them, most sea life as we know it would not exist. 

The net effect of our addition on wastewater is to make it more similar to the seawater it flows into. Since wastewater is made of freshwater and contains biological CO2 after treatment, it’s slightly more acidic than seawater. Adding magnesium hydroxide has the effect of balancing it to make it the same pH as the sea. 

As with everything, doses matter. Even water is fatal to humans in too large doses. Within the wastewater pipe, where the concentration will be the highest, it will still be less than half of the concentration required to affect a water flea (here is an example safety data sheet with the relevant information). Once it reaches the sea, it dilutes within seconds to a concentration that is undetectable by the best sensors in the world. 

Despite how safe this material has been shown to be in centuries of human use and decades of lab and open ocean experiments, we approach all research with an abundance of caution. We do this by: 

  • Testing every batch we use for trace metals and rejecting batches that don’t pass
  • Planning sediment and biological surveys before, during and after the research period.
  • Deploying sensors throughout the system from the point of addition through to the surrounding ocean.

Given all of that, let’s dig into the details! 

Safe for ocean mammals and fish

The “gold standard” of testing for aquatic animals is a test of acute levels of any substance on fish, specifically Pimephales promelas (fathead minnows) and on aquatic invertebrates (specifically Daphnia Magna or “water flea”, a small planktonic crustacean). In both of these tests (here and here, click on “Reference 1”), magnesium hydroxide is shown to not be toxic at levels that are much higher than any level considered during research projects.

Some people have asked us if the addition could have a laxative effect. For humans, a regular “laxative” dose is 3,735 mg. Based on this calculation, a female adult seal would need to drink up to 100 000L of water from within a very close proximity (~10 m) of the outfall in one sitting to get a similar laxative dose.

Safe for ocean plankton

What about plankton – the very important foundation of the food chain? 

A recently published study in Tasmania showed very limited impacts to phytoplankton communities that were subjected to prolonged alkalinity increases at more than 100 times higher concentrations than being considered for this study, and these authors consider the climatic benefits to far outweigh the concerns for these communities (Ferderer et al., 2022). 

Similar studies have been conducted at Dalhousie University in Canada and, although not yet published, preliminary results from these studies also conclude that for the phytoplankton species tested, no impacts were found in scenarios that mimic the naturally flushed systems where field additions will occur. Also noted in recent research, is that in much higher concentrations, there could be some impact if the light reaching the plankton is blocked in some way, for example if the water becomes murky as a result of large amounts of magnesium hydroxide particles. 

Even at concentration levels that are much higher than planned for our research, there is no reasonable risk of water becoming murky. Nonetheless, because plankton are so important to the ocean ecosystem, we will monitor both the turbidity of the water, and the plankton communities themselves near the diffuser very carefully to ensure that there is no unforeseen impact. 

Risks do not build 

Is there a chance that this substance is like a pesticide, similar to DDT or some other substance that slowly accumulates in plants and animals, becoming more dangerous as you move up the food chain? 

No. This question (called bio-accumulation) has been directly studied. There is no evidence that magnesium hydroxide is bioaccumulative (see here “Magnesium hydroxide has no bioaccumulation potential…”). 

Combine basic safety with low concentrations and short durations

The short duration of our research is another element of the study that creates additional safety. 

And as a final safety precaution, all research starts at a very low concentration. Milk of magnesia is produced at a concentration of 415 mg/5ml (which is 83,000mg/L). This is more than 700 times the concentration that we would be adding into the ocean. And as soon as the material reaches the ocean, it disperses very quickly, about 1000 times within 50-100 metres of the diffuser, and even more as the tide and currents wash the plume away from the area. This means that even a short way away from the diffuser, the concentration is more than 700,000 times more dilute than the milk of magnesia you buy in a store. 

For all these reasons… 

  • Magnesium hydroxide has been ingested by humans and commonly used in wastewater treatment, without trouble, for decades, 
  • Testing indicates that it is safe for aquatic animals, 
  • It does not show any bioaccumulation, and 
  • The study is for a short duration at a low concentration, 

…we are confident that our research projects are safe.Please see our blog post on monitoring to learn how we will gather data to prove that no harm occurs, and please see our answers on the blog post on the moral imperative of carbon removal to see why we believe that this is a critical tool in healing our climate.

You have stated that the carbon is locked up in the ocean for 100,000 years. How do you know that?

Our planet and its atmosphere form a closed environment. Carbon continuously cycles in and out of land and sea – into the atmosphere – and back again. The ocean plays a very important role in this cycle as it stores the vast majority of the carbon on Earth (approx 34,000 metric Gigatons or 34,000 Gt) in the form of dissolved, alkaline carbon in seawater. In addition, we also know that this carbon is stored out of the atmosphere for very long periods of time. We know this by measuring how much alkaline carbon the ocean intakes annually and by validating how much of this carbon is precipitated out annually – the net result is approx 0.3 Gt carbon loss from this alkaline carbon pool. From this we can calculate the mean time the carbon will be locked away, which is called residence time:

Residence time of carbon= (total carbon stored) (net loss of carbon  / year) 

34,000 GT / (0.3 GT / year) = 113, 333 years

We round this down to 100,000 years to be conservative and to account for uncertainties in the calculation. This storage timescale is widely accepted by the scientific community and policy makers, and is considered one of the main attractions for this type of carbon removal compared to other less permanent approaches that are currently proposed. As noted on our project page (, our process captures CO2 and converts it to this same long-lived alkaline carbon just discussed. Doing so will allow the ocean to play the important role of safely removing and sequestering carbon from the atmosphere. While the chemistry involved in this form of carbon storage is a bit complicated, it is also well published and understood. 

Lead Responder: Dr. Greg Rau
Chief Technical Officer, Co-Founder & Board Member

Please can you explain how you source Magnesium Hydroxide (MH) and how you intend to source it as you scale up?

Lead Responder: Jason Vallis
Vice President, Operations

Ultimately the goal is always to use the nearest and lowest-carbon source of MH that we possibly can. Each site is unique and careful incremental trials are required before we scale up in any substantial way. That’s how we keep things safe. As a result, it doesn’t typically make sense to invest in building a new, low carbon supply of MH nearby until we’ve gone through an incremental scale up and have the “demand” that would make that large investment worthwhile. That means that at low scales, we need to rely on the sources that are available today. 

There are a number of pathways to produce MH for carbon removal:

Mined directly from the ground as brucite, the mineral form of magnesium hydroxide. This is relatively scarce in a pure form and therefore high cost, but has a low carbon footprint as it requires little processing. Our first trials use brucite since it’s available today without needing large infrastructure to be built.

Produced through the addition of slaked lime (Ca(OH)2) to brines such as seawater. If the slaked lime is produced from limestone using a kiln that captures CO2 emissions, and the brine does not contain carbon or the carbon is kept in the brine through the process, this can be a suitable MH source for carbon removal. This could be a low cost and globally abundant source since limestone is so prevalent, but requires carbon-capture kilns (such as that produced by Origen, a UK company working in Teesside) to be deployed. 

Produced through the calcination of MgCO3 or magnesite rock with carbon capture. This process, again, requires a carbon capturing kiln. It’s got good cost potential but low scale since magnesite is less common than limestone. 

Produced through the extraction of MH from magnesium silicate rock (the process described on our website). Planetary is pioneering this process and it will be several years before it gets to scale. This process, though, has the highest potential since magnesium silicate is globally abundant and is produced at massive scale as a waste in nickel, lithium and other mining activities. 

Both from a cost and efficiency perspective, local sources of MH are preferred. Every mile of transport creates emissions which reduce the total net removals of the process and increase costs. Transportation emissions depend on the mode of transport. For bulk product shipments, the emissions, from lowest per tonne-mile to highest are: long-haul ship, short-haul ship, rail, truck, plane. That means that the most viable sources are those which are coastally located. It also means that removals can generally be achieved at various places around the world based on these coastal sources. 

Assuming this trial is successful, our ultimate goal is to source MH as near to the point of addition as possible. While none of the following sourcing arrangements are currently in place, we see the following potential sources: 

  • Development of the Planetary process to utilise magnesium silicate waste from mining operations in Cornwall
  • Provision of a carbon-capturing kiln to existing MH producers 
  • Development of new brine processing from the use of UK limestone with a carbon-capturing kiln 

Until these processes are developed and deployed, we’ll use MH from other sources on a per project basis that still provide a net carbon benefit in the process despite transportation emissions.

How do you respond to concerns regarding the potential moral hazard at play in your work?

This question strikes at the moral implications of the carbon removal – this is a hard question to address fully as a single question. Mike Kelland therefore opted to take a moment to offer a more detailed response as a blog post.

Lead Responder: Mike Kelland
Chief Executive Officer, Co-Founder & Board Member