家庭環境でのラドン被曝2013年10月29日 18:39

線量計を使って環境放射線量を調べたり、自治体等が発表しているモニタリングポストの値をこまめに見られている方はご存知と思いますが、雨が降ると空間線量は上がります。時には50%近く上昇する事もあります(雨量と線量に直接の関係は無いようです)。この線量上昇の原因がラドンや、その関連物質からのγ線である事はご存知の方も多いと思います。
ウラン238のα崩壊を始点として、最終的に安定な鉛206にまで物質変化する過程では「ウラン系列」と呼ばれる様々な元素の状態を通過します。

この長い物質変化の道のりの途中で半減期1600年のラジウム226になり、このラジウム226がα崩壊して今回の中心人物であるラドン222に変わります。気体状のラドン222の半減期は3.8日と短く、α崩壊して、ポロニウム218に変わります(暗殺事件に顔を出した悪名高いポロニウム210に変わるのは、まだ先、終点の鉛206に変わる直前です)。
ポロニウム218は、半減期がわずか3分でα崩壊し鉛214に変わります。鉛214も半減期が26分と短く、β崩壊して、ビスマス214に変わります。ビスマス214も20分と半減期が短くβ崩壊して・・・・・・最後に、やっと安定した鉛206に辿りつき、それ以降は変化しません。

このウラン系列の物質変遷の途中で生じる気体ラドンは、比重が空気の7倍と随分重たいのですが、それでも一部は気流に乗って上空大気に混じります。崩壊して鉛214やビスマス214に変わりますが、その時点では鉛やビスマスもエアロゾル(浮遊微粒子)として大気中に漂ったままです。
ここで雨が降ると、上空に漂っていたラドン222、鉛214、ビスマス214が雨滴に叩かれたり下降気流で地上に落ちてきます。それらがα崩壊、β崩壊する際に同時に発するγ線によって空間線量が上がる事になると言う訳です。

なんだか「風が吹けば桶屋が儲かる」みたいな長ーい話ですが、要は、ラドンはガスの状態で地中から滲み出て、重たいので一部は地上の窪地や家屋内などに溜まり、一部は気流に乗って上空へ昇り雨に打たれて地上に戻ってきて、空間線量を増やすと言う事なのでしょう。

日本では、このラドンをめぐって語られている多くは、
①この国の首相が、健康維持のためにラドンを一生懸命吸引している。どうも、吸うと、ハイになって元気が出るらしい。
②地震の前兆現象としてラドン濃度の急上昇が観測される(事がある)。だから、ラドン濃度の変化を見る事で次の大地震を予測できる       の2つだと思います。
多くの日本人は、この何れかの目的で「ラドン計測器」を購入して数値を見つめているのでしょう。これらのラドンの効用の真偽については次回に譲る事とします。

しかし、広く世界を見渡せば、屋内のラドンガスは肺ガンを引き起こす強い因子として警戒されている事が判ります。WHOは、屋内ラドンを喫煙に次ぐ肺癌リスクとしており、その危険性をLNTモデル(被曝量が増えるほど危険性は増す。この線量以下なら害はないといった「閾(しきい)値」は存在しないとするモデル)で説明しており、100Bq(ベクレル)/㎥(立方メートル)以下とする事を提唱しています。このWHOの勧告を受けて多くの国が屋内ラドンによる被曝対策をとっています。米国では、屋内ラドン濃度の規制値を148Bq/㎥と定めており、不動産の取引時にはラドンガスの計測と提示が義務付けられている程だそうです。我が総理大臣のようにラドンを医療目的で使用する事は米国では禁止されており、違反すると罰則があると聞きます。他の多くの国も、米国よりは緩やかではありますが、規制値を定めています。

しかし日本は規制の無い大変珍しい国です。
日本で屋内ラドンが野放しにされてきた理由は主に次の3つだと思います。
1.湿気の多い日本では、昔から風通しを重視した高床式の住宅が多く、他国に較べて屋 
  内ラドンの濃度は全国平均が15~17Bq/㎥と、もともと低かった(世界平均は40くら
  い)。
2.首相のラドン吸引の根拠となっているホルミシス効果(低線量被曝は身体に良い)という 
  考えが一部の学者・企業などによって広められた。ラドン温泉やラジウム温泉の効能に 
  ついても同様。
3.人の健康・安全をテーマにした際の、いつもながらの学者・官僚の鈍感さと無能

2や3の問題については大変面白いので次回にまとめたいと思いますが、1についてだけ補足すると、この15~17Bq/㎥というデータは木造建築で得ており、サンプリング時期も古いものが多いという事です。最近の、マンション・アパートといったコンクリート建造物ではどうなのか?佐藤満彦氏は「放射能は怖いのか」の中で「コンクリートの建物の内部では、木造家屋に比べて100倍もの放射線に曝される」と書いています。又、木造建築でもアルミサッシの普及で密閉性が上がっているはずです。

と、言う事で私も一つラドン計測器を購入して、Qベクのメンバーの皆様にも協力いただき、最新のデータがどんなものか集めてみようと考えました。

多くの方が使っているのは、多分、下の写真のような米国製かカナダ製の計測器でしょう。これは、国内で3万円弱で売られていますが、海外から直接買えば1万円くらいで入手できそうです。そこで海外のサイトでこの機械の購入方法を調べていたら、ノルウェイ製の最新機の記事に出会いました。現地価格が199ユーロ(約2万7千円)とやや高価ですが、最新のものだけあって、測定精度が高く、軽量・小型な上、米国製がACアダプターを介して家庭用コンセントからの100Vで動くのに対して、こちらは単4電池3本を使用し2年以上の連続運転が可能とあり、小さい事も相まって可搬性に勝っています。因みに開発責任者は日本人の学者との事。このノルウェイ製の測定器に決めました(運賃込みで3万ちょっとです)。
             米国製のラドン測定器(大きく交流100V仕様)

早速、ノルウェイのメーカーに発注したところ、驚いた事に、3日後には商品が手許に届きました(下の写真)。UPSと言う国際的な輸送会社の便だったのですが、最近の物流の進歩は凄いものだと感心させられました。
               到着したノルウェイ製ラドンモニター

梱包を解いて、説明書を読み「木造2階建てアルミサッシ使用」の我が家の1階での数値を調べるべく、早速動かし始めました。手順は、至って簡単・・・ただ測りたい部屋のテーブル等の上に置くだけです(但し、壁から25Cm以上、床から50Cm以上離れた場所が推奨されています)。

動かし始めて2週間経過した我が家のダイニング(1階)の平均値は11Bq/㎥と判定されました。1日の平均値で最も高かったのは17Bq/㎥、最も低い日は7Bq/㎥でした。前者は午後から気圧が下がり、夕刻から雨が降り始めた日で、後者は北寄りの弱風が1日吹く快晴の日でした。
通常、ラドン濃度は夏に高く、冬は低いと言われていますが、天候・気圧・風向風力・湿度などの要素との関係も追々調べたいと思います。まだ、たった2週間動かしただけですが、我が家1階での変動の第一要因は、何と言っても「風」だろうと推測しています。
何れにせよ、まずは全国平均の15~17Bq/㎥を少しながら下回って良かった良かった。で、これから我が家の2階の様子を調べた上で、Qベクメンバーに順次調査して貰う予定で、途中経過を含めて、報告したいと思います。

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